Stock Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Restricted Stock [Member]	Mar. 31, 2012 Restricted Stock [Member]	Mar. 31, 2013 Retiring named executive officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grant			12,500,000
Shares available for future issuance			1,500,000
Options Roll Forward
Shares outstanding under option, Beginning Balance				1,436,000
Weighted average exercise price, Beginning Balance				$ 30.65
Shares outstanding under option, Exercised	(81,000)
Shares outstanding under option, Exercised, Weighted average exercise price	$ 28.43
Shares outstanding under option, Ending Balance	1,355,000			1,436,000
Weighted average exercise price, Ending Balance	$ 30.78			$ 30.65
Weighted average remaining contractual term	3 years 2 months	3 years 5 months
Aggregate intrinsic value, outstanding	$ 63.1			$ 58.8
Restricted stock compensation expense
Restricted stock compensation expense					$ 11.1	$ 4.7	$ 5.5
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance				1,032,000
Nonvested shares subject to restriction, weighted average exercise price				$ 58.77
Restricted shares granted to employees	372,000				371,697
Granted shares subject to restriction, weighted average exercise price	$ 75.41
Shares subject to restriction, Vested	(331,000)
Vested shares subject to restriction, weighted average exercise price	$ 47.69
Shares subject to restriction, Forfeited	(19,000)
Forfeited shares subject to restriction, weighted average exercise price	$ 68.14
Nonvested shares subject to restriction, Ending Balance	1,054,000			1,032,000
Nonvested shares subject to restriction, weighted average exercise price	$ 67.71			$ 58.77