Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum payout range	200.00%
Income tax benefit	$ 0.3		$ 0.3
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0.7		0.6
Restricted Stock [Member]
Stock-Based Compensation Activity
Number of shares granted	1,912
Fair value of shares granted	$ 59.41
Common Stock
Stock-Based Compensation Activity
Number of shares issued	69,674
Restricted Stock [Member]
Stock-Based Compensation Activity
Shares Paid for Tax Withholding for Share Based Compensation	186
Performance Units Related to Earnings Per Share [Member]
Stock-Based Compensation Activity
Number of shares granted	11,785
Fair value of shares granted	$ 53.45
Equity Instruments Other than Options, Converted in Period	12,576	[1]
Performance Units Related to Earnings Per Share [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0.2		0.2
Total Shareholder Return [Member]
Stock-Based Compensation Activity
Number of shares granted	35,353
Fair value of shares granted	$ 58.61
Equity Instruments Other than Options, Converted in Period	37,724	[1]
Total Shareholder Return [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 0.5		$ 0.4

[1] Performance units were converted based on a payout ratio of 200 percent of the target number of performance units granted in February 2010 and are included in the 69,674 shares of common stock issued during the three months ended March 31, 2013 as discussed above.