Stockholders' Equity - AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance		$ 515,851
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing	$ 4,500	0	$ 4,537	$ 0
AOCI, ending balance		573,682	515,851
Net Unrealized Gain/(Loss) on AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance		585,250	813,515	752,912
OCI before reclassifications		72,560	(194,890)	95,551
Amounts reclassified from AOCI		(37,407)	(37,912)	(34,948)
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing		0	4,537	0
Net OCI during the period		35,153	(228,265)	60,603
AOCI, ending balance	813,515	620,403	585,250	813,515
Net Unrealized Gain/(Loss) on Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance		(69,399)	(59,062)	(15,217)
OCI before reclassifications		22,678	(10,337)	(44,292)
Amounts reclassified from AOCI		0	0	447
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing		0	0	0
Net OCI during the period		22,678	(10,337)	(43,845)
AOCI, ending balance	(59,062)	(46,721)	(69,399)	(59,062)
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, beginning balance		515,851	754,453	737,695
OCI before reclassifications		95,238	(205,227)	51,259
Amounts reclassified from AOCI		(37,407)	(37,912)	(34,501)
Cumulative effect adjustment on adoption of revised accounting standard for repurchase agreement financing		0	4,537	0
Net OCI during the period		57,831	(238,602)	16,758
AOCI, ending balance	$ 754,453	$ 573,682	$ 515,851	$ 754,453