Offsetting Assets and Liabilties - Offsetting of Liabilties (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	$ 3,281,168,000	$ 2,795,687,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	3,281,168,000	2,795,687,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(3,271,916,000)	(2,782,870,000)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(9,252,000)	(12,485,000)
Net Amount Of Liabilities	0	332,000
Repurchase Agreement [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	3,278,456,000	2,793,705,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	3,278,456,000	2,793,705,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(3,271,916,000)	(2,782,870,000)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(6,540,000)	(10,835,000)
Net Amount Of Liabilities	0	0
Swap [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	2,712,000	802,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	2,712,000	802,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	0	0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(2,712,000)	(802,000)
Net Amount Of Liabilities	0	0
TBA securities [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	0	1,180,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	0	1,180,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	0	0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	0	(848,000)
Net Amount Of Liabilities	$ 0	$ 332,000