Stockholders' Equity and EPS Reclassification of AOCI (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Related tax expense	$ 35,285	$ 47,404	$ 29,968	$ 26,401	$ 29,526	$ 28,470	$ 31,517	$ 23,756	$ (139,058)	$ (113,269)	$ (119,087)
Net income available to common stockholders	58,757	67,621	48,584	40,891	50,421	42,289	47,603	34,790	215,853	175,103	171,902
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 included in net income									(538)	(4,241)	(37,127)
Related tax expense									218	1,661	15,189
Net income available to common stockholders									$ (320)	$ (2,580)	$ (21,938)