Share-Based Compensation (Details 2) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Equity stock-based compensation
Expense recognized during the period	$ 4.2	$ 4.1	$ 8.5	$ 7.4
Restricted Shares and Restricted Units
Equity stock-based compensation
Vesting terms, minimum (in years)			P3Y
Vesting terms, maximum (in years)			P5Y
Expense recognized during the period	2.0	1.0	3.0	2.0
Restricted Shares
Restricted share and unit activity
Non-vested at the beginning of the period (in shares)			136
Vested (in shares)			(34)
Cancelled (in shares)			(4)
Non-vested at the end of the period (in shares)	98		98
Weighted-average fair value
Non-vested at the beginning of the period (in dollars per share)			$ 30.69
Vested (in dollars per share)			$ 33.61
Cancelled (in dollars per share)	$ 25.58		$ 25.58
Non-vested at the end of the period (in dollars per share)	$ 29.87		$ 29.87
Unrecognized compensation cost	1		1
Unrecognized compensation cost amortized (in years)			1.4
Restricted Units
Restricted share and unit activity
Non-vested at the beginning of the period (in shares)			235
Granted (in shares)			162
Vested (in shares)			(9)
Cancelled (in shares)			(6)
Non-vested at the end of the period (in shares)	382		382
Weighted-average fair value
Non-vested at the beginning of the period (in dollars per share)			$ 44.24
Granted (in dollars per share)			$ 55.96
Vested (in dollars per share)			$ 36.77
Cancelled (in dollars per share)	$ 46.60		$ 46.60
Non-vested at the end of the period (in dollars per share)	$ 49.56		$ 49.56
Unrecognized compensation cost	$ 13		$ 13
Unrecognized compensation cost amortized (in years)			2.2