Investments - Hedge funds with redemption restrictions and advance notice period requirements (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
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	70.3
Hedge funds | 1-29 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | 30-59 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.1
Hedge funds | 60-89 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	20.6
Hedge funds | 90-119 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	25.4
Hedge funds | 120+ days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	24.2
Hedge funds | Quarterly
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	44.5
Hedge funds | Quarterly | 1-29 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Quarterly | 30-59 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.1
Hedge funds | Quarterly | 60-89 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	19.8
Hedge funds | Quarterly | 90-119 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	24.6
Hedge funds | Quarterly | 120+ days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Semi-annual
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.8
Hedge funds | Semi-annual | 1-29 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Semi-annual | 30-59 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Semi-annual | 60-89 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.8
Hedge funds | Semi-annual | 90-119 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Semi-annual | 120+ days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Annual
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	25.0
Hedge funds | Annual | 1-29 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Annual | 30-59 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Annual | 60-89 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.0
Hedge funds | Annual | 90-119 days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	0.8
Hedge funds | Annual | 120+ days notice
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments, Fair value	$ 24.2