Total realized and unrealized investment gains and net investment income - Summary of total realized and unrealized investment gains and net investment income (Details) - USD ($) $ in Millions	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
Net Investment Income [Line Items]
Other investment expenses							$ (11.6)	$ (1.1)	$ (1.0)
Total realized and unrealized investment gains and net investment income	$ (151.2)	$ 199.8	$ 77.4	$ 186.5	$ 263.9	$ 463.7	312.5	278.9	282.5
Debt securities
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							(4.9)	72.7	9.2
Short-term investments
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							1.6	0.0	0.0
Equity securities
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							(2.5)	0.0	0.0
Other long-term investments
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							35.2	0.0	0.0
Related party investment funds
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							304.0	195.0	249.6
Before other investment expenses and investment income (loss) on cash and cash equivalents
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							333.4	267.7	258.8
Cash and cash equivalents
Net Investment Income [Line Items]
Realized and unrealized investment gains and net investment income before investment expenses							$ (9.3)	$ 12.3	$ 24.7