Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of provision for tax from the federal rate to the effective tax rate [Abstract]
Income tax computed at statutory tax rate	$ 54,021	$ 48,812	$ 53,008
Tax exempt interest	(7,611)	(6,780)	(5,908)
Nondeductible interest expense	407	201	119
State income taxes, net	3,697	1,754	1,408
Income tax credits, net	(15,793)	(16,183)	(15,283)
Death benefit gains	(3,268)
Reversal of valuation allowance	(8,650)
Re-measurement of deferred tax assets	25,619
Other	293	3,249	2,070
Income tax provision	$ 48,715	$ 31,053	$ 35,414