Income Taxes - Income tax rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	35.00%	35.00%	35.00%
Non-deductibility of goodwill impairment	(29.50%)	(0.00%)	(0.00%)
Change in federal tax rate	18.20%	0.00%	0.00%
State and other taxes, net of federal tax benefit	0.40%	2.70%	2.80%
Change in unrecognized tax benefits	(0.70%)	0.50%	0.80%
Change in valuation allowance	0.30%	0.00%	0.00%
Domestic production activity deduction	0.30%	(0.60%)	(0.70%)
Changes in tax rates and state tax laws	(0.40%)	(1.90%)	(0.00%)
Change in accounting for excess tax deficiencies/benefits	(0.50%)	(0.00%)	(0.00%)
Other	(0.80%)	0.30%	(0.10%)
Effective tax rate	22.30%	36.00%	37.80%