Stock-Based Compensation (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Stock-based compensation
Total unrecognized compensation cost related to the unvested awards | $	$ 14.8
Total weighted average remaining term of unrecognized compensation costs	1 year 6 months 26 days
Compensation cost recognized | $	$ 13.9	$ 13.4	$ 10.9
Class A
Stock-based compensation
Shares available for future grants of new equity awards | shares	1,342,858
Performance stock units
Stock-based compensation
Granted (in shares) | shares	98,000	107,000	106,000
Total unrecognized compensation cost related to the unvested awards | $	$ 5.3
Total weighted average remaining term of unrecognized compensation costs	1 year 6 months 4 days
Compensation cost recognized | $	$ 4.8	$ 4.0	$ 1.7
Fair value assumptions
Weighted average grant-date fair value (in dollars per share)	$ 60.45	$ 55.27	$ 58.94
Performance stock units | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 75.95
Second Amended and Restated 2004 Stock Incentive Plan
Stock-based compensation
Number of stock incentive plans | item	1
Second Amended and Restated 2004 Stock Incentive Plan | Stock options
Stock-based compensation
Total unrecognized compensation cost related to the unvested awards | $	$ 0.0
Compensation cost recognized | $	$ 0.5	$ 1.1	$ 1.9
Summary of stock option activity and related information
Outstanding at beginning of year (in shares) | shares	130,000	362,000	495,000
Cancelled/Forfeitures (in shares) | shares	(3,000)	(43,000)	(69,000)
Exercised (in shares) | shares	(32,000)	(189,000)	(64,000)
Outstanding at end of year (in shares) | shares	95,000	130,000	362,000
Exercisable at end of year (in shares) | shares	93,000	82,000	192,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 54.46	$ 48.46	$ 47.34
Cancelled/Forfeitures (in dollars per share)	55.81	52.93	51.66
Exercised (in dollars per share)	53.19	43.31	36.29
Outstanding at end of year (in dollars per share)	54.91	54.46	48.46
Exercisable at end of year (in dollars per share)	54.85	$ 53.38	$ 45.10
Weighted Average Intrinsic Value
Outstanding at end of year (in dollars per share)	21.04
Exercisable at end of year (in dollars per share)	$ 21.10
Aggregate intrinsic values of exercisable options (in dollars) | $	$ 2.0
Total intrinsic value of options exercised | $	$ 0.5	$ 3.5	$ 1.2
Second Amended and Restated 2004 Stock Incentive Plan | Stock options | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 75.95
Management Stock Purchase Plan
Stock-based compensation
Percentage of annual incentive bonus that may be used to purchase RSU's	50.00%
Purchase price as percentage of fair market value of common stock on grant date	80.00%		67.00%
Management Stock Purchase Plan | Class A
Stock-based compensation
Shares authorized | shares	2,000,000
Shares available for future grants of new equity awards | shares	763,233
Management Stock Purchase Plan | Restricted stock | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 75.95
Management Stock Purchase Plan | Restricted stock units (RSUs)
Stock-based compensation
Granted (in shares) | shares	47,000	89,000	60,000
Total unrecognized compensation cost related to the unvested awards | $	$ 1.2
Total weighted average remaining term of unrecognized compensation costs	1 year 4 months 24 days
Compensation cost recognized | $	$ 1.0	$ 0.7	$ 0.6
Fair value assumptions
Expected life (years)	3 years	3 years	3 years
Expected stock price volatility (as a percent)	25.00%	24.80%	23.40%
Expected dividend yield (as a percent)	1.20%	1.30%	1.20%
Risk-free interest rate (as a percent)	1.50%	0.90%	1.10%
Weighted average grant-date fair value (in dollars per share)	$ 16.84	$ 18.15	$ 19.04
Management Stock Purchase Plan | Restricted stock units (RSUs) | Minimum
Stock-based compensation
Vesting period	3 years