Defined Contribution Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States Pension Plans Of US Entity, Defined Contribution [Member] | UNITED STATES
Defined Contribution Plan Disclosure [Line Items]
Expense recognized	$ 28.1	$ 26.8	$ 26.5
Supplemental matching	5.5	5.4	4.8
Foreign Pension Plans, Defined Contribution [Member] | UNITED KINGDOM
Defined Contribution Plan Disclosure [Line Items]
Expense recognized	$ 3.9	$ 3.6	$ 4.0