Stockholders' Equity and EPS Reclassification of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Related tax expense (benefit)	$ (54,825)	$ (76,877)	$ (65,047)	$ (53,584)	$ (53,697)	$ (57,017)	$ (54,974)	$ (63,066)	$ (250,333)	$ (228,754)	$ (183,508)
Net income available to common stockholders	$ 99,466	$ 111,081	$ 92,964	$ 79,174	$ 87,512	$ 81,733	$ 86,143	$ 88,516	382,685	343,904	263,870
Accumulated Net Unrealized Investment Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification adjustment for (gains) losses included in net income									(12,195)	(1,201)	18,598
Related tax expense (benefit)									4,963	481	(7,510)
Net income available to common stockholders									$ (7,232)	$ (720)	$ 11,088