Employee Benefit Plans - Fair Value Hierarchy (Details) - Post-Retirement Medical Benefits - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Employee benefit plans
Total investments measured at fair value	$ 12,827	$ 11,240	$ 13,773
Fair Value
Employee benefit plans
Total investments measured at fair value	12,827	11,240
Level 1
Employee benefit plans
Total investments measured at fair value	12,827	11,240
Level 2
Employee benefit plans
Total investments measured at fair value	0	0
Level 3
Employee benefit plans
Total investments measured at fair value	0	0
Cash equivalents | Fair Value
Employee benefit plans
Total investments measured at fair value	189	215
Cash equivalents | Level 1
Employee benefit plans
Total investments measured at fair value	189	215
Cash equivalents | Level 2
Employee benefit plans
Total investments measured at fair value	0	0
Cash equivalents | Level 3
Employee benefit plans
Total investments measured at fair value	0	0
Fixed income fund | Fair Value
Employee benefit plans
Total investments measured at fair value	5,001	4,380
Fixed income fund | Level 1
Employee benefit plans
Total investments measured at fair value	5,001	4,380
Fixed income fund | Level 2
Employee benefit plans
Total investments measured at fair value	0	0
Fixed income fund | Level 3
Employee benefit plans
Total investments measured at fair value	0	0
U.S. equity securities | Fair Value
Employee benefit plans
Total investments measured at fair value	7,637	6,645
U.S. equity securities | Level 1
Employee benefit plans
Total investments measured at fair value	7,637	$ 6,645
U.S. equity securities | Level 2
Employee benefit plans
Total investments measured at fair value	0
U.S. equity securities | Level 3
Employee benefit plans
Total investments measured at fair value	$ 0