Income Taxes - Statutory Federal Tax Rate (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
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount									$ 28,564	$ 45,740	$ 41,775
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									0	1,365	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount									(946)	(860)	0
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount									0	4	21
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount									324	120	261
Income tax expense	$ 6,193	$ 7,979	$ 7,332	$ 6,438	$ 13,266	$ 11,193	$ 11,124	$ 10,786	$ 27,942	$ 46,369	$ 42,057
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent									21.00%	35.00%	35.00%