Stockholders' Equity - AOCI Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income Available to Common Stock and Participating Securities	$ 96,843	$ 60,055	$ 76,185	$ 74,310	$ 68,887	$ 79,261	$ 75,200	$ 74,320	$ 307,393	$ 297,668	$ 298,226
Total reclassifications for period									39,739	37,407	37,912
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				$ 1,700				0	1,700
Available for sale MBS
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for period									(39,739)	(37,407)	(37,912)
Amounts Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income Available to Common Stock and Participating Securities									(39,739)	(37,407)	(37,912)
Amounts Reclassified from AOCI | Available for sale MBS
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities									(38,707)	(36,922)	(37,207)
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]
OTTI recognized in earnings									$ (1,032)	$ (485)	$ (705)