Income Taxes (Effective Income Tax Reconciliation) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	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
Income Tax Expense (Benefit) Reconciliation
Income tax expense at statutory federal income tax rate										$ 114,944	$ 168,048	$ 135,957
State income tax expense, net of federal income tax benefit										17,270	11,961	13,256
Excess tax benefit from share-based compensation										(9,865)	46,573	0
Non-deductible expenses										7,556	2,435	813
Low income housing tax credits and other tax benefits										(6,421)	(2,759)	0
Low income housing tax credit amortization										5,316	268	0
Change in valuation allowance										(3,431)	(6,227)	(2,055)
Low income housing tax credit amortization										(3,055)	(4,702)	(3,402)
Low income housing tax credits and other tax benefits										(2,801)	(4,318)	0
General business tax credits										(1,163)	(4,615)	(1,213)
Other, net										528	(2,000)	(1,689)
Total income tax expense		$ 38,784	$ 18,949	$ 30,936	$ 30,209	$ 74,361	$ 54,668	$ 41,788	$ 33,847	$ 118,878	$ 204,664	$ 141,667
Effective tax rate										21.70%	42.60%	36.50%
Provisional income tax expense (benefit)											$ 608
Tax Cuts and Jobs Act of 2017, provisional income tax expense											$ 47,200
Change in tax rate, income tax expense (benefit)											9.80%
Increase in retained earnings	$ 7,600				$ 7,600						$ 7,600
Tax Cuts and Jobs Act of 2017, Income Tax Expense (Benefit)										$ (9,800)
Tax credit, research											$ 4,600