Investments - Investments in hedge funds and private equity interests by investment objective and sector (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	$ 456.1	$ 4.0
Hedge funds and private equity funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	195.7	$ 0.0
Other long-term investments, unfunded commitments	26.7
Total hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	70.3
Other long-term investments, unfunded commitments	0.0
Long/short multi-sector
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	19.8
Other long-term investments, unfunded commitments	0.0
Distressed mortgage credit
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	24.6
Other long-term investments, unfunded commitments	0.0
Private credit
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	24.2
Other long-term investments, unfunded commitments	0.0
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	1.7
Other long-term investments, unfunded commitments	0.0
Total private equity funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	125.4
Other long-term investments, unfunded commitments	26.7
Energy infrastructure & services
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	48.4
Other long-term investments, unfunded commitments	19.0
Multi-sector
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	10.1
Other long-term investments, unfunded commitments	5.1
Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	31.0
Other long-term investments, unfunded commitments	2.2
Life settlement
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	12.9
Other long-term investments, unfunded commitments	0.0
Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	19.9
Other long-term investments, unfunded commitments	0.0
Private equity secondaries
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.5
Other long-term investments, unfunded commitments	0.4
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	2.6
Other long-term investments, unfunded commitments	$ 0.0