Offsetting Assets and Liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) counterparty	Dec. 31, 2014 USD ($)
Swaps, at fair value
Gross Amounts of Recognized Assets	$ 849	$ 3,136
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	849	3,136
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(849)	(3,136)
Cash Collateral Received	0	0
Net Amount	0	0
Swaps, at fair value
Gross Amounts of Recognized Liabilities	65,420	62,198
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheets	65,420	62,198
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	0	0
Cash Collateral Pledged	(65,420)	(62,198)
Net Amount	0	0
Repurchase agreements
Gross Amounts of Recognized Liabilities	9,635,036	8,267,388
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheets	9,635,036	8,267,388
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(9,634,312)	(8,266,619)
Cash Collateral Pledged	(724)	(769)
Net Amount	0	0
Total
Gross Amounts of Recognized Liabilities	9,700,456	8,329,586
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheets	9,700,456	8,329,586
Gross Amounts Not Offset in the Consolidated Balance Sheet
Financial Instruments	(9,634,312)	(8,266,619)
Cash Collateral Pledged	(66,144)	(62,967)
Net Amount	0	0
Additional disclosures
Excess collateral	1,900	4,300
Fair value of securities pledged against the swaps	44,600	57,200
Fair value of securities pledged against the repurchase agreements	$ 11,471,000	9,934,000
Number of repurchase agreement counterparties with unconditional right of setoff | counterparty	1
Swaps
Swaps, at fair value
Gross Amounts of Recognized Assets	$ 849	3,136
Gross Amounts Offset in the Consolidated Balance Sheets	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheets	849	3,136
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(849)	(3,136)
Cash Collateral Received	0	0
Net Amount	$ 0	$ 0