Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting of Derivative Assets
Gross Amounts	$ 16,904	$ 47,888
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets presented in the Consolidated Balance Sheets	16,904	47,888
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(11,683)	(43,838)
Cash Collateral	0	0
Net Amount	5,221	4,050
Gross amounts of derivative liability, fair value, gross liability	4,346	182,158
Derivative liabilities, net amounts of assets presented in consolidated balance sheets	4,346	182,158
Financial instruments, derivative liability, at fair value	(50)	(20,500)
Cash collateral, derivative liability, at fair value	(4,270)	(161,588)
Net Amount	26	70
Gross amounts of repurchase agreements	3,251,686	2,155,644
Repurchase agreement, net amounts of assets presented in consolidated balance sheets	3,251,686	2,155,644
Financial instruments, repurchase agreements	(3,251,686)	(2,155,644)
Gross amounts of derivative liability	3,256,032	2,337,802
Derivative liability, net amounts of assets presented in consolidated balance sheets	3,256,032	2,337,802
Derivative liability, financial instruments	(3,251,736)	(2,176,144)
Derivative liability, net amount	26	70
Derivative liability, cash collateral pledged	63,300	206,600
Fair value of investments pledged against repurchase agreements	3,500,000	2,500,000
Agency RMBS Interest-Only Strips accounted for as derivatives, included in MBS
Offsetting of Derivative Assets
Gross Amounts	16,176	27,317
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets presented in the Consolidated Balance Sheets	16,176	27,317
Financial Instruments	(11,633)	(23,338)
Gross Amounts Not Offset in the Consolidated Balance Sheets
Net Amount	4,543	3,979
Derivative asset
Offsetting of Derivative Assets
Gross Amounts	728	20,571
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets presented in the Consolidated Balance Sheets	728	20,571
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(50)	(20,500)
Cash Collateral	0	0
Net Amount	$ 678	$ 71