Income Taxes - Total income tax provision (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
Current income tax provision:
Federal									$ 19,787	$ 18,644	$ 16,928
State									13,178	7,062	4,655
Total current income tax provision									32,965	25,706	21,583
Deferred income tax provision (benefit):
Federal									8,142	8,294	2,379
Effect of Tax Act					$ 5,600				(1,441)	5,633	0
State									2,574	2,493	1,253
Total deferred income tax provision (benefit)									9,275	16,420	3,632
Total income tax provision	$ 15,376	$ 7,798	$ 10,182	$ 8,884	$ 19,370	$ 10,619	$ 7,521	$ 4,616	$ 42,240	$ 42,126	$ 25,215