Income Taxes (Effective Income Tax Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit) Reconciliation
Income tax expense at statutory federal income tax rate										$ 168,048	$ 135,957	$ 125,501
State income tax expense, net of federal income tax benefit										11,961	13,256	12,870
Provisional tax adjustment related to reduction in U.S. federal statutory income tax rate										46,573	0	0
LIHTC amortization, net of tax benefits										268	0	0
Tax-exempt income										(719)	(825)	(835)
Tax benefit from share-based compensation										(4,318)	0	0
Bank-owned life insurance										(4,702)	(3,402)	(2,885)
Change in valuation allowance										(6,227)	(2,055)	(589)
General business tax credits(4)										(6,546)	(1,213)	(1,173)
Other, net										326	(51)	(398)
Total income tax expense		$ 74,361	$ 54,668	$ 41,788	$ 33,847	$ 39,519	$ 37,375	$ 33,574	$ 31,199	$ 204,664	$ 141,667	$ 132,491
Effective tax rate										42.60%	36.50%	36.90%
Tax Cuts and Jobs Act of 2017, income tax expense										$ 608
Increase in retained earnings										7,800
Research and development tax credits totaling										4,600
Income Tax Contingency [Line Items]
Tax Cuts and Jobs Act of 2017, income tax expense										$ 47,200
Tax Cuts and Jobs Act of 2017, change in effective tax rate										9.80%
Federal
Income Tax Expense (Benefit) Reconciliation
Total income tax expense		$ 47,200
Forecast
Income Tax Expense (Benefit) Reconciliation
Increase in retained earnings	$ 7,800