Stock-Based Compensation Restricted Stock Units (Details) (Restricted Stock Units (RSUs) [Member], USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 years		Dec. 31, 2011	Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost	$ 2.3		$ 2.3	$ 1.9
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	2.7
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Beginning balance	1,112,868		955,936	570,569
Incentive shares granted	330,426		336,816	400,907
Incentive share awards vested	(407,909)		(121,500)	(4,236)
Incentive share awards cancelled	(74,997)		(58,384)	(11,304)
Ending balance	960,388		1,112,868	955,936
Incentive share awards outstanding Weighted Average Contractual Term (in years)	1.10
Incentive share awards outstanding Aggregate Intrinsic Value (in millions)	$ 18.6
Vested and expected to vest	912,369	[1]
Vested and expected to vest Weighted Average Contractual Term (in years)	1.1	[1]
Incentive share awards exercisable	0
Incentive share awards exercisable Weighted Average Contractual Term (in years)	0
Incentive share awards exercisable Aggregate Intrinsic Value (in millions)	$ 0

[1] Incentive shares expected to vest based upon historical forfeitures rate