Unaudited Quarterly Results - Schedule of unaudited quarterly results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,927.8	$ 1,663.5	$ 1,608.0	$ 1,511.5	$ 1,582.4	$ 1,500.7	$ 1,470.1	$ 1,319.0	$ 6,710.8	$ 5,872.2	$ 4,860.4
Net income	$ 820.4	$ 594.7	$ 551.4	$ 478.0	$ 173.5	$ 388.3	$ 387.7	$ 248.9	$ 2,444.4	$ 1,198.5	$ 895.5
Net income per share - basic (in dollars per share)	$ 7.58	$ 5.50	$ 5.12	$ 4.44	$ 1.62	$ 3.64	$ 3.66	$ 2.36	$ 22.65	$ 11.27	$ 8.55
Net income per share - diluted (in USD per shares)	$ 7.15	$ 5.17	$ 4.82	$ 4.16	$ 1.50	$ 3.32	$ 3.34	$ 2.16	$ 21.29	$ 10.34	$ 7.70
Remeasurement of Deferred Tax Assets due to US Tax Reform					$ 326.2