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	Dec. 31, 2021	Dec. 31, 2020
Junior subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 258.2	$ 257.8
Junior subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	264.8	265.9
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	172.9	173.6
Junior subordinated debentures | Subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	85.5	85.1
Junior subordinated debentures | Subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	91.9	92.3
Senior unsecured fixed rate notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	140.3	140.2
Senior unsecured fixed rate notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	148.4	146.7
Floating rate loan stock | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	57.0	60.7
Floating rate loan stock | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 57.1	$ 61.0