Debt - Summary of Debt Obligations (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 1,244,800,000	$ 1,243,200,000
Federal Home Loan Bank Advances (FHLB)
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 155,000,000.0	155,000,000.0
Stated interest rate	1.91%
Senior Notes | 5.75% Senior Notes Due 2023
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 241,100,000	240,600,000
Stated interest rate	5.75%
Debt instrument, face amount	$ 242,300,000
Senior Notes | 5.25% Senior Notes Due 2028
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 639,900,000	638,800,000
Stated interest rate	5.25%
Debt instrument, face amount	$ 650,000,000
9% Convertible Junior Subordinated Debentures
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 208,800,000	$ 208,800,000
Stated interest rate	9.00%