Stockholders' Equity (AOCI Reclassifications) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gain on sales of residential mortgage securities									$ 61,307,000	$ 39,577,000	$ 35,837,000
Net Income Available to Common Stock and Participating Securities	$ 57,129,000	$ 83,382,000	$ 66,645,000	$ 79,645,000	$ 96,843,000	$ 60,055,000	$ 76,185,000	$ 74,310,000	286,801,000	307,393,000	297,668,000
Total reclassifications for period									(52,839,000)	(39,739,000)	(37,407,000)
Accumulated Other Comprehensive Income/(Loss)
Unrealized losses recorded in AOCI on securities for which Other-than-temporary impairments had been recognized in earnings in prior periods	$ 224,000				$ 0				224,000	0
Amounts Reclassified from AOCI | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gain on sales of residential mortgage securities									(51,580,000)	(38,707,000)	(36,922,000)
OTTI recognized in earnings									(1,259,000)	(1,032,000)	(485,000)
Net Income Available to Common Stock and Participating Securities									$ (52,839,000)	$ (39,739,000)	$ (37,407,000)