Income Taxes - Effective rate reconciliation (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Reconciliation between the statutory United States Federal income tax rate and the Company's effective income tax rate
Statutory rate	21.00%	35.00%	21.00%	35.00%	35.00%
State taxes	2.30%	1.40%	1.90%	1.40%
Changes in state tax laws	0.00%	0.50%	1.00%	0.20%
Noncontrolling interest in partially-owned subsidiaries	(1.20%)	(1.40%)	(0.40%)	(0.90%)
Impairment and other foreign losses	9.20%	0.40%	2.60%	0.50%
Changes in valuation allowance and reserves	(1.00%)	0.10%	0.60%	0.10%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(2.60%)	0.00%	(0.70%)	0.00%
Settlements with tax authorities	0.00%	0.00%	0.00%	(2.10%)
Equity compensation	(0.40%)	(0.20%)	(1.80%)	0.80%
Other, net	0.10%	1.10%	1.20%	1.00%
Effective rate	27.40%	36.90%	25.40%	36.00%