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, 2018	Dec. 31, 2017
Junior Subordinated Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 256.9	$ 256.6
Junior Subordinated Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	258.1	257.9
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	170.9	172.9
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures, Maybrooke [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	84.2	83.9
Junior Subordinated Debentures [Member] | Junior Subordinated Debentures, Maybrooke [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	87.2	85.0
Senior Unsecured Fixed Rate Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	139.8	139.6
Senior Unsecured Fixed Rate Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	140.6	141.2
Floating Rate Loan Stock [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	58.3	58.9
Floating Rate Loan Stock [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instruments	$ 57.6	$ 59.0