Income Taxes (Details)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 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)	17.50%	37.70%
Effective tax rate on income before provision for income taxes and noncontrolling interests (as a percent)	24.00%	39.00%
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