Summary of Significant Accounting Policies (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Share based compensation arrangement by share based payment award
Share based compensation expense	$ 7.4	$ 6.1	$ 15.3	$ 12.3
Reference rate of swap yield curve used to determine fair value of interest rate swaps			LIBOR
Phantom stock unit awards
Share based compensation arrangement by share based payment award
Share based compensation expense	1.3	0.6	2.5	1.0
Total compensation cost related to nonvested awards not yet recognized	14.0		14.0
Weighted average vesting period (in years)			3.4
Phantom stock unit awards | Maximum
Share based compensation arrangement by share based payment award
Vesting period (in years)			5 years
Phantom stock unit awards | Minimum
Share based compensation arrangement by share based payment award
Vesting period (in years)			4 years
Stock appreciation rights
Share based compensation arrangement by share based payment award
Share based compensation expense	1.1	0.4	2.5	0.7
Total compensation cost related to nonvested awards not yet recognized	$ 12.5		$ 12.5
Weighted average vesting period (in years)			3.1
Stock appreciation rights | Maximum
Share based compensation arrangement by share based payment award
Vesting period (in years)			4 years