Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of provision for tax from the federal rate to the effective tax rate [Abstract]
Income tax computed at statutory tax rate	$ 36,089	$ 54,021	$ 48,812
Tax exempt interest	(4,533)	(7,611)	(6,780)
Nondeductible interest expense	416	407	201
State income taxes, net	4,738	3,697	1,754
Income tax credits, net	(15,404)	(15,793)	(16,183)
Death benefit gains	(268)	(3,268)	0
Reversal of valuation allowance	0	(8,650)	0
Re-measurement of deferred tax assets	0	25,619	0
Other	1,231	293	3,249
Income tax provision	$ 22,269	$ 48,715	$ 31,053