Disclosures About Fair Value of Financial Instruments - Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 399.2	$ 453.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	369.2	419.1
Junior subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	258.7	258.6
Junior subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	241.7	253.9
Junior subordinated debentures | Trust preferred debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	172.7	172.7
Junior subordinated debentures | Trust preferred debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	157.8	165.8
Junior subordinated debentures | Subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	86.0	85.9
Junior subordinated debentures | Subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	83.9	88.1
Senior unsecured fixed rate notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	140.5	140.5
Senior unsecured fixed rate notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	127.5	112.7
Floating rate loan stock | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	0.0	54.7
Floating rate loan stock | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 0.0	$ 52.5