Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial derivatives–assets
Financial derivatives–assets, at fair value-	$ 15,479	$ 16,788	$ 20,001
Financial Instruments Available for Offset	(12,579)	(12,755)	(10,910)
Cash Collateral (Received) Pledged	(1,404)	(807)	(2,514)
Net Amount	1,496	3,226	6,577
Reverse repurchase agreements
Repurchase agreements, at fair value	38,640	73,639	61,274
Financial Instruments Available for Offset	(38,640)	(73,639)	(61,274)
Cash Collateral (Received) Pledged	0	0	0
Net Amount	0	0	0
Repurchase agreements
Reverse repurchase agreements	(1,496,931)	(2,445,300)	(1,498,849)
Financial Instruments Available for Offset	1,496,931	73,639	61,274
Cash Collateral (Received) Pledged	28,884	31,005	16,974
Net Amount	0	0	0
Financial derivatives–liabilities
Financial derivatives–liabilities, at fair value-	(24,553)	(27,621)	(20,806)
Financial Instruments Available for Offset	12,579	12,755	10,910
Cash Collateral (Received) Pledged	10,694	12,233	9,896
Net Amount	(1,280)	(2,633)	0
Securities Collateral relating to Reverse Repurchase Agreements	1,800,000	2,800,000	1,790,000
Financial Derivatives - Assets
Financial derivatives–assets
Financial derivatives–assets, at fair value-			20,001
Financial Instruments Transferred or Pledged as Collateral	0	0	0
Financial derivatives–liabilities
Excess cash collateral	4,500	4,300	100
Securities Purchased under Agreements to Resell [Member]
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	0	0	0
Reverse repurchase agreements
Repurchase agreements, at fair value			61,274
Financial Derivatives - Liabilities
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	0	0	0
Financial derivatives–liabilities
Financial derivatives–liabilities, at fair value-			20,806
Excess cash collateral	10,300	23,400	16,400
Reverse repurchase agreements
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	$ (28,884)	$ (2,340,656)	$ (1,420,601)