Fair Value Measurements Disclosure [Line Items]

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- Definition

Number of Available for Sale Securities

+ References

No definition available.

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- Definition

Number of Credit Agreement

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

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-Publisher FASB
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- Definition

For an unclassified balance sheet, amounts due from parties associated with the reporting entity as evidenced by a written promise to pay.

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- Definition

Includes other investments and securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method.

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Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May 31, 2009 Agreement	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value Measurements Disclosure [Line Items]
Fair value			$ 2,614.1
Other invested assets, at cost		$ 5.6	5.6
Number of Credit Agreements \| Agreement	2
Notes receivable from affiliate		70.0	70.0
Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Federal home loan bank stock		5.1
Other invested assets, at cost		0.5
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		2,141.6	2,192.8
Corporate Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 525.9	529.7
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities		1
Fair value		$ 43.3	$ 45.2
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$ 70.0