Offsetting Assets and Liabilties - Offsetting of Liabilties (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	$ 2,795,687,000	$ 1,798,813,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	2,795,687,000	1,798,813,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(2,782,870,000)	(1,794,816,000)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(12,485,000)	(3,997,000)
Net Amount Of Liabilities	332,000	0
Repurchase Agreement [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	2,793,705,000	1,798,813,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	2,793,705,000	1,798,813,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(2,782,870,000)	(1,794,816,000)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(10,835,000)	(3,997,000)
Net Amount Of Liabilities	0	$ 0
Interest Rate Swap [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	802,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0
Net Amount Of Liabilities Presented In The Balance Sheet	802,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(802,000)
Net Amount Of Liabilities	0
To Be Announced Contracts [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	1,180,000
Gross Amount Of Liabilties Offset In The Balance Sheet	0
Net Amount Of Liabilities Presented In The Balance Sheet	1,180,000
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(848,000)
Net Amount Of Liabilities	$ 332,000