Share-Based Compensation Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-Based Compensation Plans
Shares authorized for future grant	2,565,196
Additional share-based compensation disclosures
Proceed from option exercises	$ 28.8	$ 70.1	$ 42.1
Stock options
Number of shares
Outstanding at the beginning of the period (in shares)	1,937,241	3,405,500	4,515,560
Exercised (in shares)	(593,204)	(1,437,053)	(1,018,010)
Expired or forfeited (in shares)	(16,625)	(31,206)	(92,050)
Outstanding at the end of the period (in shares)	1,327,412	1,937,241	3,405,500
Exercisable at the end of the period (in shares)	1,104,999
Weighted Average Exercise Price
Outstanding at the beginning of the period, Weighted Average Exercise Price (in dollars per share)	$ 49.35	$ 49.12	$ 47.39
Exercised, Weighted Average Exercise Price (in dollars per share)	$ 48.58	$ 48.78	$ 41.40
Expired or forfeited, Weighted Average Exercise Price (in dollars per share)	$ 52.13	$ 51.37	$ 49.55
Outstanding at the end of the period, Weighted Average Exercise Price (in dollars per share)	$ 49.66	$ 49.35	$ 49.12
Exercisable at the end of the period, Weighted Average Exercise Price (in dollars per share)	$ 48.43
Weighted Average Remaining Contractual Term
Weighted Average Remaining Contractual Term, stock options outstanding (in years)	2 years 3 months 18 days
Weighted Average Remaining Contractual Term, stock options exercisable (in years)	2 years 2 months 12 days
Aggregate Intrinsic Value (In millions)
Aggregate Intrinsic Value, options outstanding	15.5
Aggregate Intrinsic Value, options exercisable	14.3
Vesting period (in years)	4 years
Award expiration term (in years)	7 years
Changes in non-vested stock options
Non-vested at the beginning of the period (in shares)	683,725
Forfeited (in shares)	(16,625)
Vested (in shares)	(444,687)
Non-vested at the end of the period (in shares)	222,413	683,725
Weighted Average Grant Date Fair Value
Non-vested options at the beginning of the period (in dollars per share)	$ 24.93
Forfeited (in dollars per shares)	$ 25.15
Vested (in dollars per shares)	$ 23.90
Non-vested options at the end of the period (in dollars per shares)	$ 26.98	$ 24.93
Additional share-based compensation disclosures
Total unrecognized compensation cost	23.6
Weighted average recognition period for unrecognized compensation cost (in years)	1 year 1 month 13 days
Proceed from option exercises	28.8	70.1	42.1
Total intrinsic value of all options exercised	13.5	29.0	15.5
Tax benefit realized from option exercises	$ 10.7	$ 11.5	$ 5.4
Stock options | Non-employee director
Number of shares
Outstanding at the end of the period (in shares)	104,000
Aggregate Intrinsic Value (In millions)
Vesting period (in years)	1 year
Restricted stock, 2011 grant
Restricted Shares
Restricted stock granted (in shares)				86,000
Fair value of restricted stock granted (in dollars per share)				$ 37.29
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)				86,000
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)				$ 37.29
Restricted stock, 2012 grant
Restricted Shares
Restricted stock granted (in shares)			391,050
The number of grants made during the period on the basis service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).			138,700
Fair value of restricted stock granted (in dollars per share)			$ 57.42
Share of common stock	1
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)			391,050
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)			$ 57.42
Restricted stock, 2013 grant
Restricted Shares
Restricted stock granted (in shares)		327,780
The number of grants made during the period on the basis service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).		136,225
Fair value of restricted stock granted (in dollars per share)		$ 65.75
Share of common stock	1
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)		327,780
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)		$ 65.75
Restricted stock, 2014 grant
Restricted Shares
Restricted stock granted (in shares)	349,380
The number of grants made during the period on the basis service and performance criteria other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).	136,162
Fair value of restricted stock granted (in dollars per share)	$ 71.15
Share of common stock	1
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)	349,380
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)	$ 71.15
Options with ten-year term | Non-employee director
Number of shares
Outstanding at the end of the period (in shares)	110,199
Aggregate Intrinsic Value (In millions)
Award expiration term (in years)	10 years
Restricted stock, 2010 grant
Restricted Shares
Restricted stock granted (in shares)					61,000
Fair value of restricted stock granted (in dollars per share)					$ 41.24
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)					61,000
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)					$ 41.24
Restricted stock and RSUs
Restricted Shares
Restricted stock granted (in shares)	361,210
Fair value of restricted stock granted (in dollars per share)	$ 71.14
Number of shares unvested and outstanding	846,760
Changes in non-vested restricted stock grants and RSUs
Non-vested at the beginning of the period (in shares)	771,780
Granted (in shares)	361,210
Forfeited (in shares)	(17,150)
Vested (in shares)	(269,080)
Non-vested at the end of the period (in shares)	846,760
Weighted Average Grant Date Fair Value
Non-vested at the beginning of the period (in dollars per share)	$ 59.06
Granted (in dollars per shares)	$ 71.14
Forfeited (in dollars per shares)	$ 63.23
Vested (in dollars per shares)	$ 56.00
Non-vested at the end of the period (in dollars per shares)	$ 65.10
Restricted stock | Directors Equity Plan
Restricted Shares
Restricted stock granted (in shares)	11,830	12,418	16,842
Fair value of restricted stock granted (in dollars per share)	$ 70.99	$ 67.63	$ 49.87
Changes in non-vested restricted stock grants and RSUs
Granted (in shares)	11,830	12,418	16,842
Weighted Average Grant Date Fair Value
Granted (in dollars per shares)	$ 70.99	$ 67.63	$ 49.87
Restricted stock 2010 and 2011 grant
Aggregate Intrinsic Value (In millions)
Vesting period (in years)	5 years
Restricted Shares
Number of shares unvested and outstanding	46,600
Changes in non-vested restricted stock grants and RSUs
Non-vested at the end of the period (in shares)	46,600