Investment in securities - Amortized Cost, Carrying Amount and Fair Value (Details) - USD ($)	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Available-for-sale
Amortized cost	$ 2,785,608,000	$ 2,435,752,000
Gross unrealized gains	12,549,000	1,451,000
Gross unrealized losses	(101,904,000)	(164,717,000)
Available-for-sale investments	2,696,253,000	2,272,486,000
Held-to-maturity
Amortized cost	2,992,052,000	3,240,290,000
Gross unrealized gains	857,000	0
Gross unrealized losses	(426,439,000)	(569,250,000)
Held-to-maturity, fair value	2,566,470,000	2,671,040,000
Impairments recognized in other comprehensive income for HTM investments	0	0	$ 0
US government and federal agencies
Available-for-sale
Amortized cost	2,770,749,000	2,324,841,000
Gross unrealized gains	12,549,000	1,451,000
Gross unrealized losses	(100,827,000)	(162,673,000)
Available-for-sale investments	2,682,471,000	2,163,619,000
Held-to-maturity
Amortized cost	2,992,052,000	3,240,290,000
Gross unrealized gains	857,000	0
Gross unrealized losses	(426,439,000)	(569,250,000)
Held-to-maturity, fair value	2,566,470,000	2,671,040,000
Non-US governments debt securities
Available-for-sale
Amortized cost	0	93,803,000
Gross unrealized gains	0	0
Gross unrealized losses	0	(335,000)
Available-for-sale investments	0	93,468,000
Asset-backed securities - Student loans
Available-for-sale
Amortized cost	0	40,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Available-for-sale investments	0	40,000
Residential mortgage-backed securities
Available-for-sale
Amortized cost	14,859,000	17,068,000
Gross unrealized gains	0	0
Gross unrealized losses	(1,077,000)	(1,709,000)
Available-for-sale investments	$ 13,782,000	$ 15,359,000

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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