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Stock-Based Compensation - Summary of unrecognized stock-based compensation expense (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	$ 17.1
Weighted Average Remaining Vesting Period (Years)	2 years 4 months 27 days
Performance-based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	12.3
Weighted Average Remaining Vesting Period (Years)	1 year 8 months 23 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	4.1
Weighted Average Remaining Vesting Period (Years)	0 years 8 months 4 days
Restricted Stock Units And Deferred Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	$ 0.7
Weighted Average Remaining Vesting Period (Years)	0 years 9 months 29 days