Stock-Based Compensation (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) item $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2014
Stock-based compensation
Total unrecognized compensation cost related to the unvested awards | $	$ 13.2
Total weighted average remaining term of unrecognized compensation costs	1 year 5 months 23 days
Compensation cost recognized | $	$ 12.7	$ 17.8	$ 13.8
Class A
Stock-based compensation
Shares available for future grants of new equity awards | shares	1,083,096
Performance stock units
Stock-based compensation
Granted (in shares) | shares	94,000	88,000	96,000
Total unrecognized compensation cost related to the unvested awards | $	$ 5.5
Total weighted average remaining term of unrecognized compensation costs	1 year 5 months 26 days
Compensation cost recognized | $	$ 4.3	$ 8.5	$ 5.2
Fair value assumptions
Weighted average grant-date fair value (in dollars per share)	$ 70.65	$ 77.58	$ 81.51
Performance stock units | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 121.70
Previous Stock Incentive Plan 2004
Stock-based compensation
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
Summary of stock option activity and related information
Outstanding at beginning of year (in shares) | shares	10,000	49,000	95,000
Cancelled/Forfeitures (in shares) | shares		(1,000)
Exercised (in shares) | shares	(5,000)	(38,000)	(46,000)
Outstanding at end of year (in shares) | shares	5,000	10,000	49,000
Exercisable at end of year (in shares) | shares	5,000	10,000	49,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 53.65	$ 55.25	$ 54.91
Cancelled/Forfeitures (in dollars per share)		57.47
Exercised (in dollars per share)	55.03	55.63	54.55
Outstanding at end of year (in dollars per share)	52.40	53.65	55.25
Exercisable at end of year (in dollars per share)	52.40	$ 53.65	$ 55.25
Weighted Average Intrinsic Value
Outstanding at end of year (in dollars per share)	69.30
Exercisable at end of year (in dollars per share)	$ 69.30
Aggregate intrinsic values of exercisable options (in dollars) | $	$ 0.4
Total intrinsic value of options exercised | $	$ 0.3	$ 1.3	$ 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)	$ 121.70
Second Amended and Restated 2004 Stock Incentive Plan | Restricted Stock and Deferred Shares
Stock-based compensation
Vesting period	3 years
Percentage of stock options becoming exercisable	33.00%
Second Amended and Restated 2004 Stock Incentive Plan | Performance Shares and Deferred Shares
Stock-based compensation
Minimum service period	10 years
Minimum age	55 years
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units
Stock-based compensation
Vesting period	3 years
Period of time used to calculate the compound annual growth rate	3 years
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units | Minimum
Stock-based compensation
Percent of target shares a recipient may earn	0
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units | Maximum
Stock-based compensation
Percent of target shares a recipient may earn	2
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	731,163
Management Stock Purchase Plan | Restricted stock
Stock-based compensation
Compensation cost recognized | $	$ 0.7	$ 0.8
Management Stock Purchase Plan | Restricted stock | Class A
Weighted Average Intrinsic Value
Company's closing Common Stock price (in dollars per share)	$ 121.70
Management Stock Purchase Plan | Restricted stock units (RSUs)
Stock-based compensation
Minimum deferral period	3 years
Granted (in shares) | shares	28,000	37,000	36,000
Total unrecognized compensation cost related to the unvested awards | $	$ 0.8
Total weighted average remaining term of unrecognized compensation costs	1 year 4 months 20 days
Compensation cost recognized | $			$ 1.0
Fair value assumptions
Expected life (years)	3 years	3 years
Expected stock price volatility (as a percent)	24.60%	23.30%
Expected dividend yield (as a percent)	1.10%	1.10%
Risk-free interest rate (as a percent)	0.60%	2.50%
Weighted average grant-date fair value (in dollars per share)	$ 22.36	$ 22.16	$ 21.80
Management Stock Purchase Plan | Restricted stock units (RSUs) | Minimum
Stock-based compensation
Vesting period	3 years