Share-Based Compensation - Restricted shares (Details) - Restricted shares - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restricted shares outstanding
Outstanding at the beginning of the period (in shares))	163,000
Granted (in shares)	103,465	112,203	82,722
Vested (in shares)	(62,000)
Forfeited (in shares)	(19,000)
Outstanding at the end of the period (in shares)	185,000	163,000
Weighted-Average Grant-Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 126.26
Granted (in dollars per share)	160.49
Vested (in dollars per share)	135.53
Forfeited (in dollars per share)	142.76
Outstanding at the end of the period (in dollars per share)	$ 140.56	$ 126.26
Aggregate value at grant date	$ 16.6	$ 14.8	$ 8.7
Fair value of vested shares	8.4	$ 6.3	$ 2.5
Total future compensation cost related to unvested awards	$ 18.5
Maximum
Share-Based Compensation
Vesting period	4 years
Weighted Average
Weighted-Average Grant-Date Fair Value
Recognition period	2 years 6 months 7 days