Defined Contribution Plans - Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
United States
Defined Contribution Pension and Other Postretirement Plans [Line Items]
Expense recognized	$ 25.3	$ 22.6	$ 20.7
Supplemental matching	4.3	4.0	3.6
United Kingdom
Defined Contribution Pension and Other Postretirement Plans [Line Items]
Expense recognized	$ 4.8	$ 5.0	$ 5.7