Investments in unconsolidated entities - Significant equity method investees under fair value option (Details) $ in Millions	Dec. 31, 2021 USD ($)
BE Reinsurance Limited
Schedule of Equity Method Investments [Line Items]
Fair value	$ 15.2
Ownership interest	24.90%
BioVentures Investors (Offshore) IV LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 16.6
Ownership interest	73.00%
Vyrd Holdings LLC
Schedule of Equity Method Investments [Line Items]
Fair value	$ 23.0
Ownership interest	45.00%
Diamond LS I LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 7.1
Ownership interest	15.30%
Gateway Fund LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 4.0
Ownership interest	18.10%
Monarch Alternative Capital, LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 5.7
Ownership interest	12.80%
New Energy Capital Infrastructure Credit Fund LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 20.1
Ownership interest	18.30%
New Energy Capital Infrastructure Offshore Credit Fund LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 13.4
Ownership interest	12.20%
Great Bay LLC
Schedule of Equity Method Investments [Line Items]
Fair value	$ 1.0
Ownership interest	11.10%
Parameter Climate Inc.
Schedule of Equity Method Investments [Line Items]
Fair value	$ 3.7
Ownership interest	49.00%
Tuckerman Capital V LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 11.3
Ownership interest	42.10%
Tuckerman Capital V Co-Investment I LP
Schedule of Equity Method Investments [Line Items]
Fair value	$ 8.6
Ownership interest	47.30%