Disclosures about Fair Value of Financial Instruments - Summary of Fair Values Measured on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	$ 467.5	$ 442.2
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	144.2	139.5
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	323.3	302.7
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Junior Subordinated Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	266.5	248.2
Junior Subordinated Debentures [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Junior Subordinated Debentures [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	266.5	248.2
Junior Subordinated Debentures [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Senior Unsecured Fixed Rate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	144.2	139.5
Senior Unsecured Fixed Rate Notes [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	144.2	139.5
Senior Unsecured Fixed Rate Notes [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Senior Unsecured Fixed Rate Notes [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Floating Rate Loan Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	56.8	54.5
Floating Rate Loan Stock [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Floating Rate Loan Stock [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	56.8	54.5
Floating Rate Loan Stock [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Trust Preferred Debentures [Member] | Junior Subordinated Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	174.0	163.2
Trust Preferred Debentures [Member] | Junior Subordinated Debentures [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Trust Preferred Debentures [Member] | Junior Subordinated Debentures [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	174.0	163.2
Trust Preferred Debentures [Member] | Junior Subordinated Debentures [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Junior Subordinated Debentures, Maybrooke [Member] | Junior Subordinated Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	92.5	85.0
Junior Subordinated Debentures, Maybrooke [Member] | Junior Subordinated Debentures [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	0.0	0.0
Junior Subordinated Debentures, Maybrooke [Member] | Junior Subordinated Debentures [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	92.5	85.0
Junior Subordinated Debentures, Maybrooke [Member] | Junior Subordinated Debentures [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments	$ 0.0	$ 0.0