Share-Based Compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of changes in restricted stock and related information
Compensation expense	$ 18.6	$ 19.5	$ 16.7
Restricted stock
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	875,000
Granted (in shares)	84,000
Vested (in shares)	(191,965)	(157,245)	(115,764)
Forfeited (in shares)	(20,000)
Unvested at the end of the period (in shares)	747,000	875,000
Weighted Average Grant Date Fair Value, beginning of the period (in dollars per share)	$ 50.12
Weighted Average Grant Date Fair Value Granted (in dollars per share)	$ 47.12	$ 60.56	$ 50.75
Weighted Average Grant Date Fair Value Vested (in dollars per share)	$ 46.58
Weighted Average Grant Date Fair Value Forfeited (in dollars per share)	$ 48.66
Weighted Average Grant Date Fair Value, end of the period (in dollars per share)	$ 50.73	$ 50.12
Total fair value of awards vested during the period	8.9	7.7	7.7
Unrecognized compensation cost	$ 20.6
Period for recognition of unrecognized compensation cost (in years)	3.0
Cash-settled restricted stock units
Summary of changes in restricted stock and related information
Granted (in shares)	103,000
Forfeited (in shares)	(2,000)
Unvested at the end of the period (in shares)	101,000