Stock-Based Compensation - Unvested restricted stock and deferred shares activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	$ 16.4
Total weighted average remaining term of unvested awards	1 year 4 months 2 days
Compensation cost recognized	$ 20.2	$ 18.4	$ 22.9
Performance stock units
Summary of unvested restricted stock and deferred shares activity and related information
Unvested at beginning of year (in shares)	146	178	208
Granted (in shares)	99	108	61
Vested (in shares)	(125)	(136)	(85)
Cancelled/Forfeitures (in shares)	(7)	(4)	(6)
Unvested at end of year (in shares)	113	146	178
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 107.29	$ 88.32	$ 78.06
Granted (in dollars per share)	106.37	102.98	113.37
Vested (in dollars per share)	73.62	78.82	81.50
Cancelled/Forfeitures (in dollars per share)	132.14	110.43	83.53
Unvested at end of period (in dollars per share)	$ 142.53	$ 107.29	$ 88.32
Other Disclosures
Total fair value of shares vested	$ 21.1	$ 20.4	$ 10.8
Total unrecognized compensation cost related to the unvested awards	$ 10.0
Total weighted average remaining term of unvested awards	1 year 5 months 26 days
Compensation cost recognized	$ 9.7	$ 9.8	$ 14.6
Aggregate intrinsic value of outstanding awards	$ 23.5
Performance stock units | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 208.34
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares
Summary of unvested restricted stock and deferred shares activity and related information
Unvested at beginning of year (in shares)	121	138	166
Granted (in shares)	51	64	61
Vested (in shares)	(76)	(78)	(84)
Cancelled/Forfeitures (in shares)	(3)	(3)	(5)
Unvested at end of year (in shares)	93	121	138
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 121.39	$ 97.43	$ 77.97
Granted (in dollars per share)	167.39	143.86	128.32
Vested (in dollars per share)	112.65	94.33	81.70
Cancelled/Forfeitures (in dollars per share)	139.46	120.31	93.98
Unvested at end of period (in dollars per share)	$ 149.96	$ 121.39	$ 97.43
Other Disclosures
Total fair value of shares vested	$ 12.7	$ 11.1	$ 10.5
Total unrecognized compensation cost related to the unvested awards	$ 5.8
Total weighted average remaining term of unvested awards	1 year 21 days
Compensation cost recognized	$ 9.3	$ 7.7	$ 7.6
Aggregate intrinsic value of outstanding awards	$ 19.4
Second Amended and Restated 2004 Stock Incentive Plan | Restricted stock and deferred shares | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 208.34
Management Stock Purchase Plan | Restricted stock | Class A
Other Disclosures
Company's closing Common Stock price (in dollars per share)	$ 208.34
Management Stock Purchase Plan | Restricted stock units (RSUs)
Summary of unvested restricted stock and deferred shares activity and related information
Granted (in shares)	26	29	25
Cancelled/Forfeitures (in shares)	(1)	(2)	(1)
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 57.50	$ 47.26	$ 37.49
Other Disclosures
Total unrecognized compensation cost related to the unvested awards	$ 1.7
Total weighted average remaining term of unvested awards	1 year 6 months 7 days
Compensation cost recognized	$ 1.2	$ 0.9	$ 0.7
Aggregate intrinsic value of outstanding awards	7.2
Dividend declared and unpaid	$ 0.1