Retirement Plans - Employee Retirement Savings Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			24 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
401(k) retirement savings plan
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Employee contribution of eligible compensation for matching, percent				1
Employer matching contribution, percent				5.00%
Cost recognized to match employee 401(k) contributions	$ 2.3	$ 2.1	$ 1.7
Variable compensation program
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Employer matching contribution, percent	3.00%
Employer contribution amount	$ 0.7	$ 0	$ 0