Shareholders' Equity - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		$ 2,810.2		$ 2,810.2		$ 2,857.6
Reclassification of unrealized cumulative gain on investments	$ 15.3	0.0	$ 0.0	15.3	$ 0.0
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		(393.2)		(393.2)		(288.2)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		(200.1)		(200.1)		(87.7)
Translation loss on net investment hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		(14.2)		(14.2)		(39.9)
Accumulated other comprehensive loss, tax		(15.6)		(15.6)		(23.1)
Translation loss on long-term intercompany loans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		(132.1)		(132.1)		(128.8)
Unrealized gain on investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		0.0		0.0		15.3
Accumulated other comprehensive loss, tax						3.4
Defined benefit plans | Defined benefit pension plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		(49.8)		(49.8)		(50.5)
Accumulated other comprehensive loss, tax		(27.6)		(27.6)		(27.8)
Defined benefit plans | Retiree health care plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total shareholders’ equity		3.0		3.0		3.4
Accumulated other comprehensive loss, tax		$ 1.9		$ 1.9		$ 2.0