Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial derivatives–assets
Financial derivatives–assets, at fair value-	$ 23,082	$ 15,479
Financial Instruments Available for Offset	(14,703)	(12,579)
Cash Collateral (Received) Pledged	(2,919)	(1,404)
Net Amount	5,460	1,496
Reverse repurchase agreements
Reverse repurchase agreements	96,783	38,640
Financial Instruments Available for Offset	(96,783)	(38,640)
Cash Collateral (Received) Pledged	0	0
Net Amount	0	0
Repurchase agreements
Reverse repurchase agreements	(1,909,511)	(1,496,931)
Financial Instruments Available for Offset	1,909,511	1,496,931
Cash Collateral (Received) Pledged	28,884	28,884
Net Amount	0	0
Financial derivatives–liabilities
Financial derivatives–liabilities, at fair value-	(19,438)	(24,553)
Financial Instruments Available for Offset	14,703	12,579
Cash Collateral (Received) Pledged	969	10,694
Net Amount	(3,766)	(1,280)
Securities Collateral relating to Reverse Repurchase Agreements	2,200,000	1,800,000
Financial Derivatives - Assets
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	0	0
Financial derivatives–liabilities
Excess cash collateral	17,700	4,500
Securities Purchased under Agreements to Resell [Member]
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	0	0
Financial Derivatives - Liabilities
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	0	0
Financial derivatives–liabilities
Excess cash collateral	1,800	10,300
Reverse repurchase agreements
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	$ (28,884)	$ 28,884