Stock Compensation Plans - Omnibus Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2004	Aug. 31, 2014
Stock Compensation Plans
Par value of shares of common stock (in dollars per share)	$ 0.01	$ 0.01
Stock-based compensation expense and related income tax benefits
Stock-based compensation expense	$ 6.4	$ 5.8	$ 6.5
Income tax benefit	(2.5)	(2.2)	(2.5)
Stock-based compensation, net of income tax benefit	3.9	3.6	4.0
Compensation costs related to equity awards and amounts recognized
Compensation expense recognized	$ (6.4)	$ (5.8)	$ (6.5)
Stock options awarded
Nonqualified stock options granted (in shares)	144,089
Per share weighted-average exercise price (in dollars per share)	$ 82.11
Per share weighted-average grant date fair value (in dollars per share)	$ 13.54
Number of Stock Options
Options outstanding at the beginning of the period (in shares)	530,462
Add: Options granted (in shares)	144,089
Less: Options exercised (in shares)	207,973
Less: Options forfeited/cancelled (in shares)	1,620
Options outstanding at the end of the period (in shares)	464,958	530,462
Weighted-Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 38.35
Add: Options granted (in dollars per share)	82.11
Less: Options exercised (in dollars per share)	29.88
Less: Options forfeited/cancelled (in dollars per share)	65.60
Options outstanding at the end of the period (in dollars per share)	$ 55.60	$ 38.35
Stock Options
Stock Compensation Plans
Expiration period					10 years
Vesting period					3 years
Stock-based compensation expense and related income tax benefits
Stock-based compensation expense	$ 1.9
Compensation costs related to equity awards and amounts recognized
Unrecognized compensation cost at the beginning of the period	0.6
Grant date fair value current year grants	1.9
Compensation expense recognized	(1.9)
Unrecognized compensation cost at the end of the period	$ 0.6	$ 0.6
Expected amortization period	1 year 9 months
Stock options awarded
Number of stock options to be converted (in shares)						545,000
Performance Shares and RSUs
Stock-based compensation expense and related income tax benefits
Stock-based compensation expense	$ 4.5
Compensation costs related to equity awards and amounts recognized
Unrecognized compensation cost at the beginning of the period	1.7
Grant date fair value of current year grants	5.1
Compensation expense recognized	(4.5)
Unrecognized compensation cost at the end of the period	$ 2.3	$ 1.7
Expected amortization period	1 year 8 months
Options
Fair value assumptions
Expected term in years	5 years 9 months 6 days	5 years 9 months 6 days	5 years 9 months 18 days
Risk free interest rate (as a percent)	2.10%	1.80%	1.40%
Volatility (as a percent)	22.90%	32.10%	34.40%
Dividend yield (as a percent)	3.00%	3.00%	2.00%
Nonqualified stock options
Stock options awarded
Nonqualified stock options granted (in shares)	144,089	113,935	87,930
Per share weighted-average exercise price (in dollars per share)	$ 82.11	$ 58.03	$ 59.72
Per share weighted-average grant date fair value (in dollars per share)	$ 13.54	$ 13.51	$ 16.47
Number of Stock Options
Add: Options granted (in shares)	144,089	113,935	87,930
Weighted-Average Exercise Price
Add: Options granted (in dollars per share)	$ 82.11	$ 58.03	$ 59.72
RSUs | Non-employee members of the board of directors
Stock Compensation Plans
Vesting period	1 year
Omnibus Plan
Stock Compensation Plans
Shares of common stock reserved for future issuance					3,500,000
Par value of shares of common stock (in dollars per share)					$ 0.01
2013 Omnibus Plan
Stock Compensation Plans
Shares of common stock reserved for future issuance	680,000
Additional common stock reserved for issuance subject to shareholders approval (in shares)				1,577,000
Reduction in number of shares available for future issuance (in shares)	134,000