Segment reporting - Group's Total Assets by Reporting Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Segment reporting
Cash and cash equivalents and marketable securities	$ 531,162		$ 655,923
Other assets	77,349		73,743
Due from settlement of capital provision assets	120,422	$ 122,370	164,804	$ 96,760	$ 118,451	$ 183,858
Capital provision assets	3,337,579	$ 3,184,677	5,609,949	$ 5,615,305	$ 5,544,683	$ 5,243,917
Total assets	4,202,750		6,641,172
Operating Segments
Segment reporting
Cash and cash equivalents and marketable securities	514,170		620,677
Other assets	211,636		191,657
Due from settlement of capital provision assets	120,422		164,804
Capital provision assets	2,451,274		3,912,194
Total assets	3,433,740		5,026,085
Operating Segments \| Principal Finance
Segment reporting
Cash and cash equivalents and marketable securities	498,822		599,011
Other assets	29,442		24,348
Due from settlement of capital provision assets	120,422		164,804
Capital provision assets	2,451,274		3,912,194
Total assets	3,211,195		4,811,081
Operating Segments \| Asset Management and Other Services
Segment reporting
Cash and cash equivalents and marketable securities	15,348		21,666
Other assets	182,194		167,309
Due from settlement of capital provision assets	0		0
Capital provision assets	0		0
Total assets	222,545		215,004
Reconciling items
Segment reporting
Cash and cash equivalents and marketable securities	16,992		35,246
Other assets	(134,287)		(117,914)
Due from settlement of capital provision assets	0		0
Capital provision assets	886,305		1,697,755
Total assets	$ 769,010		$ 1,615,087