Defined Contribution Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
UNITED STATES
Defined Contribution Plan Disclosure [Line Items]
Expense recognized	$ 29.8	$ 28.1	$ 26.8
Supplemental matching	6.1	5.5	5.4
UNITED KINGDOM
Defined Contribution Plan Disclosure [Line Items]
Expense recognized	$ 4.9	$ 3.9	$ 3.6