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, 2020	Dec. 31, 2019
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 172.7	$ 172.7
Junior Subordinated Debentures [Member] | Trust Preferred Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	173.6	174.0
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	84.8	84.7
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	92.3	92.5
Senior Unsecured Fixed Rate Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	140.0	140.0
Senior Unsecured Fixed Rate Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	125.9	144.2
Floating Rate Loan Stock [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	55.0	56.3
Floating Rate Loan Stock [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 55.3	$ 56.8