Income Taxes - Differences between provision for income taxes and amount computed as statutory rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Differences between the provision for income taxes and statutory federal income tax rate
Pretax income at statutory rates	$ 45,344	$ 60,492	$ 57,047
Add (deduct):
State taxes, net of federal benefit	6,765	4,139	5,013
Bank owned life insurance earnings	(747)	(1,141)	(572)
Adjustment to reserve for uncertain tax positions	80	59	(58)
Tax-exempt interest revenue	(1,229)	(1,199)	(573)
Equity compensation	(892)	(799)	976
Transaction costs	78	408	92
Tax credit investments	(29)	(89)	(149)
Change in state statutory tax rate	583	81	250
Increase in valuation allowance	0	413	567
Release of disproportionate tax effects related to de-designated cash flow hedges	0	3,400	0
Expense due to enactment of federal tax reform	0	38,198	0
Other	(138)	1,051	(257)
Total income tax expense	$ 49,815	$ 105,013	$ 62,336