Share-Based Compensation, Nonvested Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Forfeited, number of shares	(21,687)	(13,364)	(68,552)
Nonvested Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested stock, number of shares, beginning of period	318,412
Nonvested stock, weighted-average grant date fair value, beginning of period	41.08
Granted, number of shares	87,146
Granted, weighted-average grant date fair value	70.98	54.74	36.24
Vested, number of shares	(104,904)
Vested, weighted-average grant date fair value	42.53
Forfeited, number of shares	(21,687)
Forfeited, weighted-average grant date fair value	47.54
Nonvested stock, number of shares, end of period	278,967	318,412
Nonvested stock, weighted-average grant date fair value, end of period	49.37	41.08
Nonvested stock, aggregate intrinsic value	23.3
Shares vested, fair value	7.8	9.9	15.2
Compensation expense	4.4	4.7	5.6
Income tax benefit recognized in the income statement	1.6	1.7	2.1
Unrecognized compensation cost relating to nonvested stock	6.6
Unrecognized compensation cost weighted-average period of recognition	1 year 6 months
Nonvested Stock [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting Period	1 year
Nonvested Stock [Member] | Minimum [Member] | Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting Period	3 years
Nonvested Stock [Member] | Maximum [Member] | Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting Period	5 years