Income Taxes - Effective rate reconciliation (Details)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	1.80%	1.50%	1.80%	1.50%
Changes in state tax laws	2.00%	0.10%	1.20%	0.00%
Noncontrolling interest in partially-owned subsidiaries	(0.20%)	(0.70%)	(0.10%)	(0.60%)
Changes in valuation allowance and reserve	0.20%		1.00%	0.20%
Settlements with tax authorities	0.00%	0.00%	0.00%	(3.50%)
Effective Income Tax Reconciliation, Equity Compensation	(3.30%)	3.00%	(2.10%)	1.50%
Other, net	2.40%	1.80%	2.00%	1.40%
Effective rate	23.90%	40.90%	24.80%	35.50%

Effective Income Tax Reconciliation, Equity Compensation

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Effective income tax rate reconciliation, changes in state tax laws, percent

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to noncontrolling interest income (loss) exempt from income taxes.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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