Income Taxes - Income tax changes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Corporate tax rate (as a percent)		35.00%	35.00%	35.00%
Estimated U.S. federal and state tax liability related to the transition tax on repatriation of foreign earnings and profits		$ 54.8
Amount of deemed repatriation of foreign earnings and profits		992.7
Estimated income tax benefit related to remeasurement of deferred tax assets and liabilities due to change in enacted tax rate		301.6
Income tax benefit from enactment of the Act		$ 243.4
Scenario, Forecast
Income Taxes
Corporate tax rate (as a percent)	21.00%