Business Segments - Revenue and Long-Lived Assets by Geographic Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Geographic Segments
Total revenue	$ 863.0	$ 855.0	$ 845.0	$ 837.0	$ 886.0	$ 821.0	$ 845.0	$ 812.0	$ 3,400.0	$ 3,363.0	$ 3,084.0
Property and equipment, net	500.0				547.0				500.0	547.0
U.S.
Geographic Segments
Total revenue									2,195.0	2,190.0	2,118.0
Property and equipment, net	299.0				334.0				299.0	334.0
Other
Geographic Segments
Total revenue									1,205.0	1,173.0	$ 966.0
Property and equipment, net	$ 201.0				$ 213.0				$ 201.0	$ 213.0