Purchase Transaction Fee [Member] Sale Transaction Fee [Member] Common Shares [Member] Preferred Shares [Member] Cumulative Preferred Shares Auction Rate Series C Preferred [Member] Cumulative Preferred Shares 5. 875 Serie A Preferred [Member] Cumulative Preferred Shares Auction Rate Series E Preferred [Member] Cumulative Preferred Shares 5. 000 Serie G Preferred [Member] Cumulative Preferred Shares 5. 000 Serie H Preferred [Member] Cumulative Preferred Shares 5. 450 Serie J Preferred [Member] Cumulative Preferred Shares 5. 000 Serie K Preferred [Member] Cumulative Preferred Shares 4. 250 Serie M Cumulative Preferred [Member] Cumulative Preferred Shares 5. 875 Serie D Preferred [Member] How The Fund Manages Risk [Member] Common Share [Member] Series C Preferred Shares [Member] Series D Preferred Shares [Member] Series E Preferred Shares [Member] Series G Preferred Shares [Member] Series H Preferred Shares [Member] Series J Preferred Shares [Member] Series K Preferred Shares [Member] Series M Preferred Shares [Member] Series N Preferred Shares [Member] Preferred Share [Member] Subscription Rights [Member]