Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 313.7	$ 389.9
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	286.8	325.6
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	0.0	48.3
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	26.9	13.2
All other jurisdictions
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 0.0	$ 2.8