Income Taxes - Total income tax provision (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax provision:
Federal									$ 18,644	$ 16,928	$ 12,460
State									7,062	4,655	4,144
Total current income tax provision									25,706	21,583	16,604
Deferred income tax provision (benefit):
Federal									8,294	2,379	(887)
Effect of Tax Act									5,633	0	0
State									2,493	1,253	(508)
Total deferred income tax provision (benefit)									16,420	3,632	(1,395)
Total income tax provision	$ 19,370	$ 10,619	$ 7,521	$ 4,616	$ 7,238	$ 5,877	$ 6,358	$ 5,742	$ 42,126	$ 25,215	$ 15,209