Income Taxes - Provision for income taxes and reconciliation of statutory federal income tax rate with effective income tax rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current taxes:
Federal	$ 54,071	$ 73,167	$ 72,711
State	625	7,720	7,174
Foreign	1		17
Total current taxes	54,697	80,887	79,902
Deferred Income Tax Expense (Benefit)	783	20,481	1,982
Provision for income taxes	$ 55,480	$ 101,368	$ 81,884
Reconciliation of statutory federal income tax rate with the entity's effective income tax rate
Statutory federal income tax rate (as a percent)	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit (as a percent)	2.40%	2.20%	2.00%
Share-based compensation (as a percent)	1.80%	3.40%
Effects of U.S. tax rate decrease (as a percent)	(0.40%)	2.20%
Uncertain tax positions (as a percent)	(2.20%)	(0.20%)	(0.10%)
Valuation allowance on losses capital in nature (as a percent)		(1.00%)	(3.20%)
Other items (as a percent)	0.70%	(0.30%)	0.40%
Effective income tax rate (as a percent)	23.30%	41.30%	34.10%