Stockholders' Equity (AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 3,383,952	$ 3,416,101	$ 3,261,636
OCI before reclassifications	367,840	(3,007)	(136,097)
Amounts reclassified from AOCI	(660,594)	(47,234)	(52,839)
Other Comprehensive Loss	(292,754)	(50,241)	(188,936)
Ending balance	2,524,802	3,383,952	3,416,101
Net Unrealized Gain/(Loss) on AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	392,722	417,167	620,648
OCI before reclassifications	420,281	20,335	(150,642)
Amounts reclassified from AOCI	(733,396)	(44,780)	(52,839)
Other Comprehensive Loss	(313,115)	(24,445)	(203,481)
Ending balance	79,607	392,722	417,167
Net  Gain/(Loss) on Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(22,675)	3,121	(11,424)
OCI before reclassifications	(50,127)	(23,342)	14,545
Amounts reclassified from AOCI	72,802	(2,454)	0
Other Comprehensive Loss	22,675	(25,796)	14,545
Ending balance	0	(22,675)	3,121
Net Unrealized Gain/(Loss) on Financing Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0
OCI before reclassifications	(2,314)
Amounts reclassified from AOCI	0
Other Comprehensive Loss	(2,314)
Ending balance	(2,314)	0
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	370,047	420,288	609,224
Ending balance	$ 77,293	$ 370,047	$ 420,288