Balance Sheet Offsetting - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Reverse Repurchase Agreements
Net Amounts of Assets Presented in the Balance Sheet	$ 105,660
Total
Gross Amounts of Recognized Assets	1,416,304	$ 153,365
Gross Amounts Offset in the Balance Sheet	(393)
Net Amounts of Assets Presented in the Balance Sheet	1,415,911	153,365
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(1,414,530)	(152,899)
Net Amount	1,381	466
Institutional counterparties
Securities Borrowed:
Gross Amounts of Recognized Assets	1,307,741	152,899
Net Amounts of Assets Presented in the Balance Sheets	1,307,741	152,899
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(1,307,741)	(152,899)
Interest Rate Swap and Swaptions | Institutional counterparties
Derivatives
Gross Amounts of Recognized Assets	1,526	425
Gross Amounts Offset in the Balance Sheets	(393)
Net Amounts of Assets Presented in the Balance Sheets	1,133	425
Gross Amounts Not Offset in the Balance Sheets
Net Amount	1,133	425
Reverse repurchase agreements | Institutional counterparties
Reverse Repurchase Agreements
Gross Amounts of Recognized Assets	105,660
Net Amounts of Assets Presented in the Balance Sheet	105,660
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(105,412)
Net Amount	248
Forward MBS Derivatives | Institutional counterparties
Derivatives
Gross Amounts of Recognized Assets	1,377	41
Net Amounts of Assets Presented in the Balance Sheets	1,377	41
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	$ (1,377)
Net Amount		$ 41