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Income Taxes - Reconciliation of U.S. Federal Statutory Tax Rate to Company's Effective Tax Rate (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax provision
Statutory U.S. federal tax rate		35.00%	35.00%	35.00%
State income taxes, net of federal benefit		2.70%	2.90%	2.60%
Effect of foreign operations		(5.40%)	(4.60%)	(3.50%)
Equity compensation		(0.70%)	(0.40%)	0.80%
Effect of changes in tax law, rates		(25.20%)	(0.30%)	(0.80%)
Other		0.40%	0.00%	(0.50%)
Effective tax rate (controlling interest)		6.80%	32.60%	33.60%
Non-Controlling Interests
Income tax provision
Effect of income from non-controlling interests		(1.30%)	(8.40%)	(9.20%)
Controlling interests
Income tax provision
Effective tax rate (controlling interest)	[1]	6.80%	32.60%	33.60%
Effective tax rate		5.50%	24.20%	24.40%
[1] Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).

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A ratio calculated by dividing the amount of income tax expense attributable to the controlling interest by pretax income from continuing operations attributable to the controlling interest.

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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 income tax rates.

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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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