Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial derivatives–assets
Financial derivatives–assets, at fair value-	$ 11,150	$ 20,001
Financial Instruments Available for Offset	(7,170)	(10,910)
Cash Collateral (Received) Pledged	(1,217)	(2,514)
Net Amount	2,763	6,577
Reverse repurchase agreements
Repurchase agreements, at fair value	49,641	61,274
Financial Instruments Available for Offset	(49,641)	(61,274)
Cash Collateral (Received) Pledged	0	0
Net Amount	0	0
Repurchase agreements
Reverse repurchase agreements	1,715,506	1,498,849
Financial Instruments Available for Offset	49,641	61,274
Cash Collateral (Received) Pledged	16,156	16,974
Net Amount	0	0
Financial derivatives–liabilities
Financial derivatives–liabilities, at fair value-	(27,805)	(20,806)
Financial Instruments Available for Offset	7,170	10,910
Cash Collateral (Received) Pledged	16,372	9,896
Net Amount	(4,263)	0
Securities Collateral relating to Reverse Repurchase Agreements	2,010,000	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
Financial derivatives–liabilities
Excess cash collateral	2,000	100
Securities Purchased under Agreements to Resell [Member]
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	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
Financial derivatives–liabilities
Financial derivatives–liabilities, at fair value-		20,806
Excess cash collateral	17,200	16,400
Reverse repurchase agreements
Financial derivatives–assets
Financial Instruments Transferred or Pledged as Collateral	$ 1,649,709	$ 1,420,601