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	Sep. 30, 2020	Dec. 31, 2019
Junior subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 257.7	$ 257.4
Junior subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	265.9	266.5
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	173.6	174.0
Junior subordinated debentures | Subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	85.0	84.7
Junior subordinated debentures | Subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	92.3	92.5
Senior unsecured fixed rate notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	140.1	140.0
Senior unsecured fixed rate notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	147.3	144.2
Floating rate loan stock | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	59.1	56.3
Floating rate loan stock | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 59.5	$ 56.8