Investments - Summary of debt securities (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 2,099,300,000	$ 91,400,000
Gross unrealized gains	2,700,000	10,900,000
Gross unrealized losses	(15,600,000)	(1,000,000.0)
Net foreign currency gains (losses)	(800,000)	0
Fair value	2,085,600,000	101,300,000
Securities sold, not yet purchased, at fair value	0	12,000,000.0
Asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	512,600,000	1,200,000
Gross unrealized gains	900,000	100,000
Gross unrealized losses	(400,000)	0
Net foreign currency gains (losses)	0	0
Fair value	513,100,000	1,300,000
Residential mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	306,500,000	8,100,000
Gross unrealized gains	0	600,000
Gross unrealized losses	(4,600,000)	0
Net foreign currency gains (losses)	0	0
Fair value	301,900,000	8,700,000
Commercial mortgage-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	148,400,000
Gross unrealized gains	600,000
Gross unrealized losses	(1,700,000)
Net foreign currency gains (losses)	0
Fair value	147,300,000
Bank debt
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost		300,000
Gross unrealized gains		100,000
Gross unrealized losses		0
Net foreign currency gains (losses)		0
Fair value		400,000
Corporate debt securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	605,500,000	29,400,000
Gross unrealized gains	600,000	8,400,000
Gross unrealized losses	(3,500,000)	(100,000)
Net foreign currency gains (losses)	0	0
Fair value	602,600,000	37,700,000
U.S. government and government agency
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	388,100,000	52,400,000
Gross unrealized gains	100,000	1,700,000
Gross unrealized losses	(2,800,000)	(900,000)
Net foreign currency gains (losses)	0	0
Fair value	385,400,000	53,200,000
Securities sold, not yet purchased, at fair value		12,000,000.0
Non-U.S. government and government agency
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	135,400,000
Gross unrealized gains	300,000
Gross unrealized losses	(2,600,000)
Net foreign currency gains (losses)	(800,000)
Fair value	132,300,000
U.S. states, municipalities and political subdivision
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	200,000
Gross unrealized gains	0
Gross unrealized losses	0
Net foreign currency gains (losses)	0
Fair value	200,000
Preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	2,600,000
Gross unrealized gains	200,000
Gross unrealized losses	0
Net foreign currency gains (losses)	0
Fair value	2,800,000
U.S. government and government agency
Debt and Equity Securities, FV-NI [Line Items]
Securities sold, not yet purchased, at fair value	$ 0	$ 12,000,000