Share-Based Compensation, Nonvested Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Forfeited, number of shares	(38,266)	(4,447)	(21,687)
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	202,060
Nonvested stock, weighted-average grant date fair value, beginning of period	63.00
Granted, number of shares	75,541
Granted, weighted-average grant date fair value	105.04	103.38	70.98
Vested, number of shares	(82,642)
Vested, weighted-average grant date fair value	54.53
Forfeited, number of shares	(38,266)
Forfeited, weighted-average grant date fair value	66.45
Nonvested stock, number of shares, end of period	156,693	202,060
Nonvested stock, weighted-average grant date fair value, end of period	86.89	63.00
Nonvested stock, aggregate intrinsic value	19.1
Shares vested, fair value	8.5	12.2	7.8
Compensation expense	4.0	5.6	4.4
Income tax benefit recognized in the income statement	1.5	2.1	1.6
Unrecognized compensation cost relating to nonvested stock	7.2
Unrecognized compensation cost weighted-average period of recognition	1 year 10 months 24 days
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