Stock Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2013 Restricted Stock [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2013 Restricted Stock [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2013 Retiring named executive officers [Member]	Jun. 30, 2013 Director [Member] Restricted Stock [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	1,355,000	1,436,000
Weighted average exercise price, Beginning Balance	$ 30.78	$ 30.65
Shares outstanding under option, Exercised	(201,000)	(81,000)
Shares outstanding under option, Exercised, Weighted average exercise price	$ 27.56	$ 28.43
Shares outstanding under option, Ending Balance	1,154,000	1,355,000	1,436,000
Weighted average exercise price, Ending Balance	$ 31.34	$ 30.78	$ 30.65
Weighted average remaining contractual term	3 years 1 month	3 years 2 months	3 years 5 months
Aggregate Intrinsic Value	$ 63.2	$ 63.1	$ 58.8
Restricted stock compensation expense
Restricted stock compensation expense				$ 4.8	$ 5.2	$ 15.9	$ 9.9	$ 5.5
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance	1,054,000	1,032,000
Nonvested shares subject to restriction, weighted average exercise price	$ 67.71	$ 58.77
Restricted shares granted to employees	15,000	372,000				373,197			14,140
Granted shares subject to restriction, weighted average exercise price	$ 75.32	$ 75.41
Shares subject to restriction, Vested	(65,000)	(331,000)
Vested shares subject to restriction, weighted average exercise price	$ 72.93	$ 47.69
Shares subject to restriction, Forfeited	(12,000)	(19,000)
Forfeited shares subject to restriction, weighted average exercise price	$ 75.68	$ 68.14
Nonvested shares subject to restriction, Ending Balance	992,000	1,054,000	1,032,000
Nonvested shares subject to restriction, weighted average exercise price	$ 67.37	$ 67.71	$ 58.77