Balance Sheet Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Total
Gross Amounts of Recognized Assets	$ 2,065,518	$ 153,365
Net Amounts of Assets Presented in the Balance Sheet	2,065,518	153,365
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(2,063,969)	(152,899)
Net Amount	1,549	466
Repurchase Agreements
Net Amounts of Assets Presented in the Balance Sheet	67,227
Institutional counterparties
Securities Borrowed:
Gross Amounts of Recognized Assets	1,990,598	152,899
Net Amounts of Assets Presented in the Balance Sheets	1,990,598	152,899
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(1,990,598)	(152,899)
Forward MBS Derivatives | Institutional counterparties
Derivatives
Gross Amounts of Recognized Assets	6,239	41
Net Amounts of Assets Presented in the Balance Sheets	6,239	41
Gross Amounts Not Offset in the Balance Sheets
Financial Instruments	(6,239)
Net Amount		41
Interest Rate Swap and Swaptions | Institutional counterparties
Derivatives
Gross Amounts of Recognized Assets	1,454	425
Net Amounts of Assets Presented in the Balance Sheets	1,454	425
Gross Amounts Not Offset in the Balance Sheets
Net Amount	1,454	425
Reverse repurchase agreements | Institutional counterparties
Repurchase Agreements
Gross Amounts of Recognized Assets	67,227
Net Amounts of Assets Presented in the Balance Sheet	67,227
Financial Instruments	67,132
Net Amount	$ 95