Income Taxes (Effective Income Tax Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
U.S. federal income tax rate									35.00%
Income Tax Expense (Benefit) Reconciliation
Income tax expense at statutory federal income tax rate									$ 125,501	$ 105,896	$ 88,420
State income tax expense, net of federal income tax effect									12,870	8,014	9,877
Tax-exempt income									(835)	(1,076)	(1,407)
Tax credits									(1,173)	(1,123)	(1,473)
Cash surrender value of life insurance									(2,885)	(2,928)	(2,932)
Change in valuation allowance, federal and state									(589)	(2,273)	(4,083)
Other, net									(398)	800	4,843
Total income tax expense	$ 32,342	$ 36,058	$ 32,242	$ 31,849	$ 25,756	$ 25,868	$ 27,078	$ 28,608	$ 132,491	$ 107,310	$ 93,245