Stockholders' Equity (Accumulated Other Comprehensive Income/(Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of accumulated other comprehensive income/(loss)
OCI before reclassifications	$ (28,888)		$ (57,276)
Amounts reclassified from AOCI	(9,455)	$ (14,935)	(25,580)	$ (31,315)
Net OCI during the period	(38,343)	(2,156)	(82,856)	56,544
Total AOCI
Components of accumulated other comprehensive income/(loss)
Balance at beginning of period	564,711	632,382	609,224	573,682
OCI before reclassifications		12,779		87,859
Amounts reclassified from AOCI		(14,935)		(31,315)
Net OCI during the period		(2,156)		56,544
Balance at end of period	526,368	630,226	526,368	630,226
Net Unrealized Gain/(Loss) on AFS Securities
Components of accumulated other comprehensive income/(loss)
Balance at beginning of period	548,551	668,223	620,648	620,403
OCI before reclassifications	(34,278)	6,988	(90,250)	71,188
Amounts reclassified from AOCI	(9,455)	(14,935)	(25,580)	(31,315)
Net OCI during the period	(43,733)	(7,947)	(115,830)	39,873
Balance at end of period	504,818	660,276	504,818	660,276
Net (Loss)/Gain on Swaps
Components of accumulated other comprehensive income/(loss)
Balance at beginning of period	16,160	(35,841)	(11,424)	(46,721)
OCI before reclassifications	5,390	5,791	32,974	16,671
Amounts reclassified from AOCI	0	0	0	0
Net OCI during the period	5,390	5,791	32,974	16,671
Balance at end of period	$ 21,550	$ (30,050)	$ 21,550	$ (30,050)