v3.26.1
Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Earned premiums:    
Net earned premiums $ 384.9 $ 395.0
Loss and LAE:    
Net incurred loss and LAE 207.0 261.8
P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Written premiums:    
Direct 357.3 416.8
Assumed 733.6 690.8
Gross written premiums 1,090.9 1,107.6
Ceded (500.8) (379.9)
Net written premiums 590.1 727.7
Earned premiums:    
Direct 266.7 246.3
Assumed 236.8 214.1
Gross earned premiums 503.5 460.4
Ceded (129.7) (102.4)
Net earned premiums 373.8 358.0
Loss and LAE:    
Gross 264.0 323.2
Ceded (57.3) (89.7)
Net incurred loss and LAE 206.7 233.5
Effects of Reinsurance Footnotes [Abstract]    
Ceded premiums written 500.8 379.9
Ceded premiums earned 129.7 102.4
Ceded 57.3 89.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark    
Written premiums:    
Ceded (0.0) (37.5)
Earned premiums:    
Ceded (2.7) (12.0)
Loss and LAE:    
Ceded 0.3 20.2
Effects of Reinsurance Footnotes [Abstract]    
Ceded premiums written 0.0 37.5
Ceded premiums earned 2.7 12.0
Ceded $ (0.3) $ (20.2)