Business Segments - Revenue by Geographic Location (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
Revenues from External Customers and Long-Lived Assets [Line Items]
As reported	$ 483.7	$ 420.8	$ 386.1	$ 344.5	$ 405.0	$ 349.6	$ 328.8	$ 267.1	$ 1,635.1	$ 1,350.5	$ 1,107.3
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
As reported									967.1	807.7	600.9
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
As reported									$ 668.0	$ 542.8	$ 506.4