Disclosures About Fair Value of Financial Instruments - Summary of Fair Values Measured on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	$ 383.3	$ 381.9
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	130.5	132.7
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	252.8	249.2
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	252.8	249.2
Junior subordinated debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	252.8	249.2
Junior subordinated debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures | Trust preferred debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	168.1	165.9
Junior subordinated debentures | Trust preferred debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures | Trust preferred debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	168.1	165.9
Junior subordinated debentures | Trust preferred debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures | Subordinated debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	84.7	83.3
Junior subordinated debentures | Subordinated debentures | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Junior subordinated debentures | Subordinated debentures | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	84.7	83.3
Junior subordinated debentures | Subordinated debentures | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Senior unsecured fixed rate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	130.5	132.7
Senior unsecured fixed rate notes | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	130.5	132.7
Senior unsecured fixed rate notes | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	0.0	0.0
Senior unsecured fixed rate notes | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments	$ 0.0	$ 0.0