Business Segments - Revenue and Long-Lived Assets by Geographic Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Geographic Segments
Total revenue	$ 823.0	$ 768.9	$ 766.3	$ 725.4	$ 752.2	$ 720.0	$ 729.2	$ 682.0	$ 3,083.6	$ 2,883.4	$ 2,758.8
Property and equipment, net	568.2				612.2				568.2	612.2
U.S.
Geographic Segments
Total revenue									2,118.1	1,959.0	2,144.0
Property and equipment, net	390.1				461.1				390.1	461.1
Other
Geographic Segments
Total revenue									965.5	924.4	$ 614.8
Property and equipment, net	$ 178.1				$ 151.1				$ 178.1	$ 151.1