Unaudited Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Investments, amortized cost	$ 3,691,050		$ 3,410,992
Investments in two sigma funds, cost	$ 1,471,985		$ 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)	98,522,131	98,614,386	99,029,434	98,361,499	100,140,859	101,466,997
Common stock outstanding (in shares)	98,522,131	98,614,386	99,029,434	98,361,499	100,140,859	101,466,997
Total fixed maturities
Investments, amortized cost	$ 3,267,687		$ 3,210,940
Short-term investments
Investments, amortized cost	$ 423,363		$ 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)	85,157,932	84,677,932	84,677,932	83,577,932	83,577,932	80,205,911
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	66,278,404	65,890,659	66,305,707	64,537,772	66,317,132	64,271,249
Common stock outstanding (in shares)	66,278,404	65,890,659	66,305,707	64,537,772	66,317,132	64,271,249
Common Class C
Common stock authorized (in shares)	14,923,649	15,403,649	15,403,649	16,003,649	16,003,649	19,375,670
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock issued (in shares)	14,923,649	15,403,649	15,403,649	16,003,649	16,003,649	19,375,670
Common stock outstanding (in shares)	14,923,649	15,403,649	15,403,649	16,003,649	16,003,649	19,375,670