Stock-Based Compensation (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) item $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 $ / shares shares	Dec. 31, 2014
Stock-based compensation
Total unrecognized compensation cost related to the unvested awards | $	$ 17.4
Total weighted average remaining term of unrecognized compensation costs	1 year 7 months 10 days
Compensation cost recognized | $	$ 13.8	$ 13.9	$ 13.4
Class A
Stock-based compensation
Shares available for future grants of new equity awards | shares	1,232,137
Performance stock units
Stock-based compensation
Granted (in shares) | shares	96,000	98,000	107,000
Total unrecognized compensation cost related to the unvested awards | $	$ 6.6
Total weighted average remaining term of unrecognized compensation costs	1 year 6 months 4 days
Compensation cost recognized | $	$ 5.2	$ 4.8	$ 4.0
Fair value assumptions
Weighted average grant-date fair value (in dollars per share)	$ 81.51	$ 60.45	$ 55.27
Performance stock units | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 64.53
Previous Stock Incentive Plan 2004
Stock-based compensation
Vesting period				4 years	3 years
Percentage of stock options becoming exercisable				25.00%	33.30%
Expiration period (years)				10 years
Minimum exercise price as percentage of fair market value of common stock on grant date					100.00%
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
Summary of stock option activity and related information
Outstanding at beginning of year (in shares) | shares	95,000	130,000	362,000
Cancelled/Forfeitures (in shares) | shares		(3,000)	(43,000)
Exercised (in shares) | shares	(46,000)	(32,000)	(189,000)
Outstanding at end of year (in shares) | shares	49,000	95,000	130,000	362,000
Exercisable at end of year (in shares) | shares	49,000	93,000	82,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 54.91	$ 54.46	$ 48.46
Cancelled/Forfeitures (in dollars per share)		55.81	52.93
Exercised (in dollars per share)	54.55	53.19	43.31
Outstanding at end of year (in dollars per share)	55.25	54.91	54.46	$ 48.46
Exercisable at end of year (in dollars per share)	55.25	$ 54.85	$ 53.38
Weighted Average Intrinsic Value
Outstanding at end of year (in dollars per share)	9.28
Exercisable at end of year (in dollars per share)	$ 9.28
Aggregate intrinsic values of exercisable options (in dollars) | $	$ 0.5
Total intrinsic value of options exercised | $	$ 0.5	$ 3.5
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)	$ 64.53
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%
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	750,421
Management Stock Purchase Plan | Restricted stock | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 64.53
Management Stock Purchase Plan | Restricted stock units (RSUs)
Stock-based compensation
Granted (in shares) | shares	36,000	47,000	89,000
Total unrecognized compensation cost related to the unvested awards | $	$ 0.8
Total weighted average remaining term of unrecognized compensation costs	1 year 2 months 16 days
Compensation cost recognized | $	$ 1.0	$ 1.0	$ 0.7
Fair value assumptions
Expected life (years)	3 years	3 years	3 years
Expected stock price volatility (as a percent)	24.10%	25.00%	24.80%
Expected dividend yield (as a percent)	1.00%	1.20%	1.30%
Risk-free interest rate (as a percent)	2.40%	1.50%	0.90%
Weighted average grant-date fair value (in dollars per share)	$ 21.80	$ 16.84	$ 18.15
Management Stock Purchase Plan | Restricted stock units (RSUs) | Minimum
Stock-based compensation
Vesting period	3 years