Senior Notes and Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total Corporation Subordinated Debt	$ 1,148.1	$ 1,112.4
Long-Term Debt Qualifying as Risk-Based Capital	$ 1,099.5	1,099.5
Redemption price, percentage	100.00%
Corporation Subordinated Notes
Debt Instrument [Line Items]
Total Corporation Subordinated Debt	$ 1,148.1	1,112.4
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Instrument [Line Items]
Total Corporation Subordinated Debt	$ 798.7	763.1
Interest rate	3.95%
Notes issued at a discount	0.114%
Corporation Subordinated 3.375% Notes due May 2032
Debt Instrument [Line Items]
Total Corporation Subordinated Debt	$ 349.4	349.3
Interest rate	3.375%
Debt instrument, basis spread on variable rate	1.131%
Senior Notes
Debt Instrument [Line Items]
Debt issue costs	$ 2.2
Secured Debt
Debt Instrument [Line Items]
Debt issue costs	$ 1.3
Fixed Rate Due Nov. 2020
Debt Instrument [Line Items]
Interest rate	3.45%
Notes issued at a discount	0.117%
Fixed Rate Due Aug. 2021
Debt Instrument [Line Items]
Interest rate	3.38%
Notes issued at a discount	0.437%
Fixed Rate Due Aug. 2022
Debt Instrument [Line Items]
Interest rate	2.38%
Notes issued at a discount	0.283%
Fixed Rate Due Aug. 2028
Debt Instrument [Line Items]
Interest rate	3.65%
Notes issued at a discount	0.125%
Fixed Rate Due May. 2029
Debt Instrument [Line Items]
Notes issued at a discount	0.094%
Subordinated Debt \| Interest Rate Swaps
Debt Instrument [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts	$ 126.9	$ 29.3

Discount in percentage terms at which long term debt was issued.

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- Definition

Long-Term Debt Qualifying as Risk Based Capital

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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