Senior Notes and Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Long-Term Debt	$ 2,065.0	$ 2,066.2
Federal Home Loan Bank (FHLB) Advances	2,000.0	0.0
Long-Term Debt	4,065.0	2,066.2
Long-Term Debt Qualifying as Risk-Based Capital	$ 1,490.8	1,648.5
Redemption price, percentage	100.00%
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Instrument [Line Items]
Interest rate	3.95%
Long-Term Debt	$ 720.3	720.1
Corporation Subordinated 3.375% Notes due May 2032
Debt Instrument [Line Items]
Interest rate	3.375%
Long-Term Debt	$ 349.7	349.7
Debt instrument, basis spread on variable rate	1.131%
Corporation Subordinated Notes 6.125% Due November 2032
Debt Instrument [Line Items]
Interest rate	6.125%
Long-Term Debt	$ 995.0	996.4
FHLB Fixed Rate Advance due December 2025
Debt Instrument [Line Items]
Interest rate	5.13%
Federal Home Loan Bank (FHLB) Advances	$ 30.0	0.0
FHLB Fixed Rate Advance due December 2025
Debt Instrument [Line Items]
Interest rate	5.18%
Federal Home Loan Bank (FHLB) Advances	$ 570.0	0.0
FHLB Fixed Rate Advance due March 2026
Debt Instrument [Line Items]
Interest rate	5.13%
Federal Home Loan Bank (FHLB) Advances	$ 600.0	0.0
FHLB Fixed Rate Advance due June 2026
Debt Instrument [Line Items]
Interest rate	5.09%
Federal Home Loan Bank (FHLB) Advances	$ 800.0	0.0
Subordinated Debt | Interest Rate Swaps
Debt Instrument [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts	$ 0.0	$ (29.3)