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, 2019	Dec. 31, 2018	Dec. 31, 2017
Current taxes:
Federal	$ 37,283	$ 54,071	$ 73,167
State	7,144	625	7,720
Foreign		1
Total current taxes	44,427	54,697	80,887
Deferred Income Tax Expense (Benefit)	(3,009)	783	20,481
Provision for income taxes	$ 41,418	$ 55,480	$ 101,368
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%	21.00%	35.00%
State income taxes, net of federal tax benefit (as a percent)	3.40%	2.40%	2.20%
Share-based compensation (as a percent)	1.40%	1.80%	3.40%
Effects of U.S. tax rate decrease (as a percent)		(0.40%)	2.20%
Uncertain tax positions (as a percent)	(0.20%)	(2.20%)	(0.20%)
Valuation allowance on losses capital in nature (as a percent)			(1.00%)
Other items (as a percent)	0.60%	0.70%	(0.30%)
Effective income tax rate (as a percent)	26.20%	23.30%	41.30%