Business Segments - Revenue and Long-Lived Assets by Geographic Segment (Details) - USD ($) $ 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
Geographic Segments
Total revenue	$ 885.7	$ 821.0	$ 844.7	$ 811.8	$ 823.0	$ 768.9	$ 766.3	$ 725.4	$ 3,363.2	$ 3,083.6	$ 2,883.4
Property and equipment, net	547.0				568.2				547.0	568.2
U.S.
Geographic Segments
Total revenue									2,190.5	2,118.1	1,959.0
Property and equipment, net	334.5				390.1				334.5	390.1
Other
Geographic Segments
Total revenue									1,172.7	965.5	$ 924.4
Property and equipment, net	$ 212.5				$ 178.1				$ 212.5	$ 178.1