Unaudited Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Investments, amortized cost	$ 3,489,236		$ 3,410,992
Investments in two sigma funds, cost	$ 1,574,091		$ 1,355,563
Common stock authorized (in shares)	150,000,000	150,000,000	150,000,000	150,000,000	150,000,000	150,000,000
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	98,614,386	99,273,252	99,029,434	100,140,859	101,711,441	101,466,997
Common stock outstanding (in shares)	98,614,386	99,273,252	99,029,434	100,140,859	101,711,441	101,466,997
Total fixed maturities
Investments, amortized cost	$ 3,133,507		$ 3,210,940
Short-term investments
Investments, amortized cost	$ 355,729		$ 200,052
Common Class A
Common stock authorized (in shares)	26,444,807	26,444,807	26,444,807	26,944,807	26,944,807	26,944,807
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	17,320,078	17,320,078	17,320,078	17,820,078	17,820,078	17,820,078
Common stock outstanding (in shares)	17,320,078	17,320,078	17,320,078	17,820,078	17,820,078	17,820,078
Common Class B
Common stock authorized (in shares)	84,677,932	84,677,932	84,677,932	83,577,932	81,705,911	80,205,911
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	65,890,659	66,549,525	66,305,707	66,317,132	66,015,693	64,271,249
Common stock outstanding (in shares)	65,890,659	66,549,525	66,305,707	66,317,132	66,015,693	64,271,249
Common Class C
Common stock authorized (in shares)	15,403,649	15,403,649	15,403,649	16,003,649	17,875,670	19,375,670
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	15,403,649	15,403,649	15,403,649	16,003,649	17,875,670	19,375,670
Common stock outstanding (in shares)	15,403,649	15,403,649	15,403,649	16,003,649	17,875,670	19,375,670