Employee Benefit Plans - Fair value of plan assets by asset category (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair value of plan assets by asset category
Aggregate fair value of plan assets	$ 52,081	$ 45,694
Level 1
Fair value of plan assets by asset category
Aggregate fair value of plan assets	941	932
Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	51,140	44,762
Cash
Fair value of plan assets by asset category
Aggregate fair value of plan assets	941	932
Cash | Level 1
Fair value of plan assets by asset category
Aggregate fair value of plan assets	941	932
U.K. and other international equities
Fair value of plan assets by asset category
Aggregate fair value of plan assets	13,297	11,400
U.K. and other international equities | Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	13,297	11,400
U.K. government and corporate bonds
Fair value of plan assets by asset category
Aggregate fair value of plan assets	2,671	2,529
U.K. government and corporate bonds | Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	2,671	2,529
Diversified growth fund
Fair value of plan assets by asset category
Aggregate fair value of plan assets	7,846	8,417
Diversified growth fund | Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	7,846	8,417
Liability driven investments
Fair value of plan assets by asset category
Aggregate fair value of plan assets	26,488	20,258
Liability driven investments | Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	26,488	20,258
Multi-asset credit fund
Fair value of plan assets by asset category
Aggregate fair value of plan assets	838	2,158
Multi-asset credit fund | Level 2
Fair value of plan assets by asset category
Aggregate fair value of plan assets	$ 838	$ 2,158