Disclosures About Fair Value of Financial Instruments - Schedule of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 371.4	$ 369.2
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	388.0	381.9
Junior subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	242.7	241.2
Junior subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	257.6	249.2
Junior subordinated debentures | Trust preferred debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	161.7	160.8
Junior subordinated debentures | Trust preferred debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	172.7	165.9
Junior subordinated debentures | Subordinated debentures | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	81.0	80.4
Junior subordinated debentures | Subordinated debentures | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	84.9	83.3
Senior unsecured fixed rate notes | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	128.7	128.0
Senior unsecured fixed rate notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments	$ 130.4	$ 132.7