Income Taxes - Statutory Federal Tax Rate (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 Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount									$ 45,740	$ 41,775	$ 37,827
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									1,365	0	0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount									(860)	0	0
Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount									0	0	(1,874)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount									0	0	(1,901)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount									4	21	33
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount									120	261	154
Income tax expense	$ 13,266	$ 11,193	$ 11,124	$ 10,786	$ 15,793	$ 10,529	$ 8,400	$ 7,335	$ 46,369	$ 42,057	$ 34,239
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent									35.00%	35.00%	35.00%