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Investments (Available-For-Sale Fixed Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 3,849,715		$ 3,758,720
Allowance for Expected Credit Losses	3,403	$ 1,602	555	$ 560	$ 419	$ 427
Gross Unrealized Gains	25,397		10,122
Gross Unrealized Losses	185,577		274,690
Estimated Fair Value	3,686,132		3,493,597
U.S. Treasury obligations
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	261,442		259,834
Allowance for Expected Credit Losses	0		0
Gross Unrealized Gains	701		165
Gross Unrealized Losses	10,842		16,474
Estimated Fair Value	251,301		243,525
U.S. Government-sponsored enterprise obligations
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	16,575		19,752
Allowance for Expected Credit Losses	0		0
Gross Unrealized Gains	20		2
Gross Unrealized Losses	571		1,030
Estimated Fair Value	16,024		18,724
State and municipal bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	482,509		482,367
Allowance for Expected Credit Losses	0		0
Gross Unrealized Gains	4,807		1,885
Gross Unrealized Losses	19,537		29,871
Estimated Fair Value	467,779		454,381
Corporate debt
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,850,831		1,883,308
Allowance for Expected Credit Losses	2,608	1,210	0
Gross Unrealized Gains	10,442		4,025
Gross Unrealized Losses	88,245		136,759
Estimated Fair Value	1,770,420		1,750,574
Residential mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	534,605		481,267
Allowance for Expected Credit Losses	201	$ 203	211	$ 216	$ 223	$ 229
Gross Unrealized Gains	5,118		2,876
Gross Unrealized Losses	45,427		53,795
Estimated Fair Value	494,095		430,137
Agency commercial mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	8,022		9,369
Allowance for Expected Credit Losses	0		0
Gross Unrealized Gains	14		5
Gross Unrealized Losses	785		987
Estimated Fair Value	7,251		8,387
Other commercial mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	212,274		210,469
Allowance for Expected Credit Losses	406		151
Gross Unrealized Gains	548		60
Gross Unrealized Losses	12,027		20,904
Estimated Fair Value	200,389		189,474
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	483,457		412,354
Allowance for Expected Credit Losses	188		193
Gross Unrealized Gains	3,747		1,104
Gross Unrealized Losses	8,143		14,870
Estimated Fair Value	$ 478,873		$ 398,395