Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes
U.S. federal statutory income tax rate (as a percent)		21.00%	35.00%	35.00%
Effective tax rate on income before provision for income taxes including income attributable to noncontrolling interests (as a percent)		21.60%	35.10%
Effective tax rate on income before provision for income taxes and noncontrolling interests (as a percent)		24.50%	36.50%
Income tax expense related to 2017 tax act and changes related to foreign tax credits	$ 0.8
Minimum period of statute of limitations of state and foreign jurisdictions		3 years
Maximum period of statute of limitations of state and foreign jurisdictions		5 years