Stockholders' Equity (Accumulated Other Comprehensive Income/(Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Components of accumulated other comprehensive income/(loss)
Balance	$ 2,521,051	$ 3,403,373	$ 3,383,952	$ 3,416,101
OCI before reclassifications	(7,570)		335,806	19,701
Amounts reclassified from AOCI	7,116	(15,184)	(660,594)	(38,139)
Other Comprehensive Income/(Loss)	(454)	(9,934)	(324,788)	(18,438)
Balance	2,565,656	3,403,426	2,565,656	3,403,426
Total  AOCI
Components of accumulated other comprehensive income/(loss)
Balance	45,713	411,784	370,047	420,288
OCI before reclassifications		5,250
Amounts reclassified from AOCI		(15,184)
Other Comprehensive Income/(Loss)		(9,934)
Balance	45,259	401,850	45,259	401,850
Net Unrealized Gain/(Loss) on AFS Securities
Components of accumulated other comprehensive income/(loss)
Balance	52,889	439,898	392,722	417,167
OCI before reclassifications	15,082	5,483	408,585	50,085
Amounts reclassified from AOCI	(60)	(14,499)	(733,396)	(36,370)
Other Comprehensive Income/(Loss)	15,022	(9,016)	(324,811)	13,715
Balance	67,911	430,882	67,911	430,882
Net (Loss)/Gain on Swaps
Components of accumulated other comprehensive income/(loss)
Balance	(7,176)	(28,114)	(22,675)	3,121
OCI before reclassifications	0	(233)	(50,127)	(30,384)
Amounts reclassified from AOCI	7,176	(685)	72,802	(1,769)
Other Comprehensive Income/(Loss)	7,176	(918)	22,675	(32,153)
Balance	0	$ (29,032)	0	$ (29,032)
Net Unrealized Gain/(Loss) on Financing Agreements
Components of accumulated other comprehensive income/(loss)
Balance	0		0
OCI before reclassifications	(22,652)		(22,652)
Amounts reclassified from AOCI	0		0
Other Comprehensive Income/(Loss)	(22,652)		(22,652)
Balance	$ (22,652)		$ (22,652)