Retirement Benefit Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Retirement Benefit Plan
Employer match of employee contributions of first 1% of eligible compensation (as a percent)	100.00%
Percentage of eligible compensation, matched 100% by employer	1.00%
Employer match of employee contributions of next 5% of eligible compensation (as a percent)	50.00%
Percentage of eligible compensation, matched 50% by employer	5.00%
Recognized matching contributions from retirement benefit plans (in dollars)	$ 8.2	$ 7.5	$ 8.6