Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 54,900	$ 50,800	$ 45,700
Increase (decrease) in tax resulting from:
Tax-exempt income	(4,500)	(4,400)	(4,100)
State income tax, net of federal income tax benefit	4,900	4,300	3,800
Benefit of stock-based compensation plans	(2,600)	0	0
Federal tax credits	(2,400)	(2,100)	(2,300)
Benefit due to enactment of federal tax reform	(2,200)	0	0
Other, net	2,200	1,000	600
Total income tax expense	$ 50,201	$ 49,623	$ 43,662