Balance Sheet Offsetting - Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Total
Gross Amounts of Recognized Liabilities	$ 2,060,258	$ 1,498,296
Gross Amounts Offset in the Balance Sheet	(23)	(14)
Net Amounts of Liabilities Presented in the Balance Sheet	2,060,235	1,498,282
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(2,006,947)	(1,455,191)
Net Amount	53,288	43,091
Institutional counterparties
Securities loaned:
Gross Amounts of Recognized Liabilities	1,447,570	1,283,676
Net Amounts of Liabilities Presented in the Balance Sheets	1,447,570	1,283,676
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,393,258)	(1,237,868)
Net Amount	54,312	45,808
Institutional counterparties | Interest Rate Swap and Swaptions
Derivatives:
Gross Amounts of Recognized Liabilities	118	297
Gross Amounts Offset in the Balance Sheet	(23)	(14)
Net Amounts of Liabilities Presented in the Balance Sheet	95	283
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(1,149)	(3,000)
Net Amount	(1,054)	(2,717)
Institutional counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	476,181	39,970
Net Amounts of Liabilities Presented in the Balance Sheet	476,181	39,970
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(476,181)	(39,970)
Net Amount	0	0
Institutional counterparties | Forward MBS Derivatives
Derivatives:
Gross Amounts of Recognized Liabilities	839	19,159
Net Amounts of Liabilities Presented in the Balance Sheet	839	19,159
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(839)	(19,159)
Net Amount	0	0
Institutional counterparties | Interest Rate Options
Derivatives:
Gross Amounts of Recognized Liabilities	30
Net Amounts of Liabilities Presented in the Balance Sheet	30
Gross Amounts Not Offset in the Balance Sheet
Net Amount	30
Customer counterparties | Repurchase agreements
Repurchase agreements:
Gross Amounts of Recognized Liabilities	135,520	155,194
Net Amounts of Liabilities Presented in the Balance Sheet	135,520	155,194
Gross Amounts Not Offset in the Balance Sheet
Financial Instruments	(135,520)	(155,194)
Net Amount	$ 0	$ 0