Senior Notes and Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Corporation-Subordinated Debt	$ 2,081.3	$ 2,065.0
Federal Home Loan Bank (FHLB) Advances	2,000.0	2,000.0
Long-Term Debt	4,081.3	4,065.0
Long-Term Debt Qualifying as Risk-Based Capital	$ 1,347.1	1,490.8
Redemption price, percentage	100.00%
Fixed Rate Notes due October 2025
Debt Instrument [Line Items]
Interest rate	3.95%
Corporation-Subordinated Debt	$ 736.0	720.3
Fixed-to-Floating Rate Notes due May 2032
Debt Instrument [Line Items]
Interest rate	3.375%
Corporation-Subordinated Debt	$ 349.8	349.7
Debt instrument, basis spread on variable rate	1.131%
Fixed Rate Notes due November 2032
Debt Instrument [Line Items]
Interest rate	6.125%
Corporation-Subordinated Debt	$ 995.5	995.0
FHLB Fixed Rate Advance due December 2025
Debt Instrument [Line Items]
Interest rate	5.13%
Federal Home Loan Bank (FHLB) Advances	$ 30.0	30.0
FHLB Fixed Rate Advance due December 2025
Debt Instrument [Line Items]
Interest rate	5.18%
Federal Home Loan Bank (FHLB) Advances	$ 570.0	570.0
FHLB Fixed Rate Advance due March 2026
Debt Instrument [Line Items]
Interest rate	5.13%
Federal Home Loan Bank (FHLB) Advances	$ 600.0	600.0
FHLB Fixed Rate Advance due June 2026
Debt Instrument [Line Items]
Interest rate	5.09%
Federal Home Loan Bank (FHLB) Advances	$ 800.0	$ 800.0