Debt and letter of credit facilities - Summary of debt obligations (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total debt	$ 816.7	$ 114.3
Subordinated Notes
Debt Instrument [Line Items]
Debt, at face value	303.1
Unamortized discount	(6.8)
Total debt	$ 296.3
Effective rate	4.10%
Senior Notes \| 2016 SIG Senior Notes
Debt Instrument [Line Items]
Debt, at face value	$ 400.0
Unamortized premium	6.0
Total debt	$ 406.0
Effective rate	4.50%
Senior Notes \| 2015 TPRUSA Senior Notes
Debt Instrument [Line Items]
Debt, at face value	$ 115.0	115.0
Unamortized issuance costs	(0.6)	(0.7)
Total debt	$ 114.4	$ 114.3
Effective rate	7.00%	7.00%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt premium.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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