Schedule I - Summary of investments - Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2021 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	$ 3,360.5
Fair value	3,362.1
Balance sheet value	3,362.1
Asset-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	512.6
Fair value	513.1
Balance sheet value	513.1
Residential mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	306.5
Fair value	301.9
Balance sheet value	301.9
Commercial mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	148.4
Fair value	147.3
Balance sheet value	147.3
Corporate debt securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	605.5
Fair value	602.6
Balance sheet value	602.6
U.S. government and government agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	388.1
Fair value	385.4
Balance sheet value	385.4
Non-U.S. government and government agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	135.4
Fair value	132.3
Balance sheet value	132.3
U.S. states, municipalities and political subdivision
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	0.2
Fair value	0.2
Balance sheet value	0.2
Preferred stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	2.6
Fair value	2.8
Balance sheet value	2.8
Total debt securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	2,099.3
Fair value	2,085.6
Balance sheet value	2,085.6
Total short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	1,076.0
Fair value	1,075.8
Balance sheet value	1,075.8
Total equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	4.5
Fair value	2.8
Balance sheet value	2.8
Other long-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	180.7
Fair value	197.9
Balance sheet value	$ 197.9