Stockholders' Equity (AOCI Reclassifications) (Details) - USD ($)	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gain on sales of residential mortgage securities										$ 62,002,000	$ 61,307,000	$ 39,577,000
Net income available to common stock and participating securities	$ 96,871,000	$ 91,849,000	$ 89,290,000	$ 85,107,000	$ 57,129,000	$ 83,382,000	$ 66,645,000	$ 79,645,000		362,030,000	285,858,000	306,502,000
Total reclassifications for period										(47,234,000)	(52,839,000)	(39,739,000)
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	$ 0				$ 224,000					0	224,000
Amounts Reclassified from AOCI | AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Including Noncontrolling Interest
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gain on sales of residential mortgage securities										(44,600,000)	(51,580,000)	(38,707,000)
OTTI recognized in earnings										(180,000)	(1,259,000)	(1,032,000)
Net income available to common stock and participating securities										(44,780,000)	(52,839,000)	(39,739,000)
Amortization of de-designated hedging instruments										(2,454,000)
Total Swaps designated as cash flow hedges										$ (2,454,000)
Amounts Reclassified from AOCI | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of de-designated hedging instruments									$ 0			0
Total Swaps designated as cash flow hedges											$ 0	$ 0