Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Taxes
Effective tax rate (as a percent)	30.40%	28.20%	31.50%	25.70%
Decrease in tax position due to settlement of previously uncertain tax position				$ 17.6
Decrease in effective tax rate due to settlement of previously uncertain tax positions (as a percent)				5.30%
Income tax at U.S. federal statutory tax rate (as a percent)			35.00%	35.00%