Senior Notes and Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total corporation subordinated debt	$ 1,145.7	$ 1,189.3
Long-Term Debt Qualifying as Risk-Based Capital	$ 799.8	949.7
Redemption price, percentage	100.00%
Corporation Subordinated Notes
Debt Instrument [Line Items]
Total corporation subordinated debt	$ 1,145.7	1,189.3
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Instrument [Line Items]
Interest rate	3.95%
Total corporation subordinated debt	$ 796.2	839.8
Corporation Subordinated 3.375% Notes due May 2032
Debt Instrument [Line Items]
Interest rate	3.375%
Total corporation subordinated debt	$ 349.5	349.5
Debt instrument, basis spread on variable rate	1.131%
Secured Debt
Debt Instrument [Line Items]
Debt issue costs	$ 0.9
Subordinated Debt \| Interest Rate Swaps
Debt Instrument [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts	$ 46.9	$ 90.8

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