Total realized and unrealized investment gains and net investment income - Summary of net realized and unrealized investment gains (losses) (Details) - USD ($)	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Gross realized gains							$ 40,400,000	$ 64,300,000	$ 9,100,000
Gross realized losses							(9,600,000)	(15,900,000)	(9,000,000.0)
Net realized investment gains							30,800,000	48,400,000	100,000
Net unrealized gains (losses) on investments							(47,700,000)	20,800,000	15,200,000
Net realized and unrealized gains (losses) on investments	$ (60,600,000)	$ (11,700,000)	$ 23,900,000	$ 31,500,000	$ 55,400,000	$ 43,700,000	(16,900,000)	69,200,000	15,300,000
Realized gains (losses) from related party investments							12,900,000	0	0
Unrealized gains (losses) from related party investments							$ (5,800,000)	$ 0	$ 0