Income Taxes - Reconciliation (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
Income Taxes
Statutory tax rate (as a percent)									35.00%	35.00%	35.00%
Computed at the statutory rate (35%)									$ 27,006	$ 19,178	$ 16,513
Increase resulting from
State income taxes									2,399	2,264	2,062
Effect of Tax Cuts and Jobs Act									(6,928)
Other										226	223
Income tax expense	$ 1,330	$ 6,445	$ 9,091	$ 5,611	$ 5,728	$ 7,587	$ 4,986	$ 3,367	$ 22,477	$ 21,668	$ 18,798