Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of provision for tax from the federal rate to the effective tax rate [Abstract]
Income tax computed at statutory tax rate	$ 53,008	$ 56,732	$ 53,899
Tax exempt interest	(5,908)	(5,612)	(5,222)
Nondeductible interest expense	119	107	121
State income taxes, net	1,408	2,933	2,604
Income tax credits	(15,283)	(15,212)	(15,755)
Other	2,070	(419)	1,290
Income tax provision	$ 35,414	$ 38,529	$ 36,937