Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Summary of unaudited consolidated pro forma results of operations
Revenues	$ 5,755.8	$ 6,003.1	$ 11,321.5	$ 11,833.1
Income from continuing operations	117.7	134.6	217.6	243.0
Net income	$ 117.8	$ 134.7	$ 217.8	$ 243.2
Income from continuing operations per diluted common share (in dollars per share)	$ 1.42	$ 1.58	$ 2.60	$ 2.84
Net income per diluted common share (in dollars per share)	$ 1.42	$ 1.59	$ 2.60	$ 2.84