Income Taxes - Difference Between Applicable Income Taxes and the Amount Derived by Applying Statutory (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Computed tax expense at statutory federal rate									$ 16,371	$ 20,036	$ 18,972
Tax-exempt income									(470)	(600)	(758)
State tax (net of federal tax benefit)									874	303	425
Non-deductible merger expense									0	455	0
Excess tax benefit – stock based compensation									(848)	(1,049)	(565)
Adjustment to net deferred tax assets for enacted changes in tax laws, rates and return to provision adjustments									(1,895)	15,193	0
Other, net									133	(108)	94
Total income tax expense	$ 1,746	$ 4,066	$ 3,723	$ 4,630	$ 19,924	$ 4,766	$ 4,905	$ 4,635	$ 14,165	$ 34,230	$ 18,168
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal rate (as a percent)									21.00%	35.00%	35.00%
Tax-exempt income (as a percent)									(0.60%)	(1.00%)	(1.40%)
State tax (net of federal tax benefit) (as a percent)									1.10%	0.50%	0.80%
Non-deductible merger expense (as a percent)									0.00%	0.80%	0.00%
Excess tax benefit – stock based compensation (as a percent)									(1.10%)	(1.80%)	(1.00%)
Adjustment to net deferred tax assets (as a percent)									(2.40%)	26.50%	0.00%
Other, net (as a percent)									0.20%	(0.20%)	0.10%
Total income tax expense (as a percent)									18.20%	59.80%	33.50%