Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Summary of unaudited consolidated pro forma results of operations
Revenues	$ 6,013.7	$ 5,996.3	$ 17,888.9	$ 18,384.2
Income from continuing operations	115.5	117.9	319.7	338.0
Net income	$ 115.7	$ 118.0	$ 320.4	$ 338.0
Income from continuing operations per diluted common share (in dollars per share)	$ 1.41	$ 1.39	$ 3.86	$ 3.96
Net income per diluted common share (in dollars per share)	$ 1.42	$ 1.39	$ 3.86	$ 3.96