Balance Sheet Offsetting - Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Reverse repurchase agreements:
Net Amounts of Assets Presented in the Balance Sheet	$ 134,654	$ 89,430
Total
Gross Amounts of Recognized Assets	1,758,136	1,546,865
Gross Amounts Offset in the Balance Sheet	(1,170)	(3,893)
Net Amounts of Assets Presented in the Balance Sheet	1,756,966	1,542,972
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,696,342)	(1,484,045)
Net Amount	60,624	58,927
Institutional counterparties
Securities borrowed:
Gross Amounts of Recognized Assets	1,611,885	1,436,069
Net Amounts of Assets Presented in the Balance Sheet	1,611,885	1,436,069
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,554,629)	(1,385,664)
Net Amount	57,256	50,405
Interest Rate Options | Customer counterparties
Derivatives:
Gross Amounts of Recognized Assets	30
Net Amounts of Assets Presented in the Balance Sheet	30
Gross Amounts Not Offset in the Balance Sheet
Net Amount	30
Reverse repurchase agreements | Institutional counterparties
Reverse repurchase agreements:
Gross Amounts of Recognized Assets	134,654	89,430
Net Amounts of Assets Presented in the Balance Sheet	134,654	89,430
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(134,040)	(89,369)
Net Amount	614	61
Forward MBS Derivatives | Institutional counterparties
Derivatives:
Gross Amounts of Recognized Assets	11,567	21,366
Gross Amounts Offset in the Balance Sheet	(1,170)	(3,893)
Net Amounts of Assets Presented in the Balance Sheet	10,397	17,473
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(7,673)	(9,012)
Net Amount	$ 2,724	$ 8,461