Stockholders' Equity and EPS - Reclassification of AOCI (Details) - USD ($) $ in Thousands	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 on Derivatives [Line Items]
Reclassification adjustment for losses (gains) included in net income (1)									$ 88,094		$ 64,603		$ 51,740
Related tax (benefit) expense (1)	$ (105,000)	$ (95,308)	$ (77,287)	$ (73,966)	$ (135,051)	$ (97,351)	$ (71,656)	$ (51,405)	(351,561)	[1]	(355,463)	[1]	(250,333)
Net income available to common stockholders	$ 266,264	$ 274,817	$ 237,798	$ 194,961	$ 117,210	$ 148,620	$ 123,193	$ 101,483	973,840	[1]	490,506	[1]	382,685
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 losses (gains) included in net income (1)									740		5,189		(12,195)
Related tax (benefit) expense (1)									(205)		(2,098)		4,963
Net income available to common stockholders									$ 535		$ 3,091		$ (7,232)

[1] Included in income tax expense, net income available to common stockholders, earnings per common share-basic and earnings for common share-diluted, for the years ended December 31, 2018 and 2017, are tax benefits recognized associated with the adoption of Accounting Standards Update ("ASU") 2016-09, Improvements to Employee Share-Based Payment Accounting in the first quarter of 2017. This guidance was adopted on a prospective basis with no change to prior period amounts.