Available-for-Sale Securities Other-Than-Temporary Impairments in Accumulated Other Comprehensive Income (Loss)

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Collateral Requirements, Minimum Percentage, Collateral To Fair Value Of Securities Loaned Or Purchased Under Repurchase Agreements

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Amount of non-binding funding commitments for private placement securities.

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Committments to Fund Commercial Mortgage Loans

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Financing Receivable, 90 Days or More Past Due, Count

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Loan To Value Ratio Maximum

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Loan to Value Ratio Update Frequency

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Mortage Loan Policy Debt Service coverage ratio lower range

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Mortgage Loan Inspection Frequency

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Mortgage loan policy, amortization period

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Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans

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Available for Sale Securities Continuous Unrealized Loss Position, 12 Months Or Longer, Number of Positions

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Unfunded Commitments and Standby Letters of Credit

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Number of financing receivables that have been modified by troubled debt restructurings.

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Amount, before troubled debt restructuring, of financing receivable to be modified.

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

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Amount of expense related to estimated loss from loan and lease transactions.

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This describes the time frame of the terms of the entity's repurchase agreements.

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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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Amount, after the effects of master netting arrangements, of securities borrowed from entities in exchange for collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

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Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended
Investments - Additional Information (Detail)	Sep. 30, 2019 USD ($) Integer	Sep. 30, 2018 USD ($) Integer	Sep. 30, 2019 USD ($) Integer	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Investments
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0		$ 0
Commitment to Fund Private Placement Fixed Maturity Securities	173,600,000		173,600,000
Carrying Amount of Variable Interest Entity Investments	623,900,000		623,900,000		575,300,000
Unfunded Commitments to Investment Partnerships	2,000,000.0		2,000,000.0
Fixed Maturity Securities	$ 47,435,000,000.0		$ 47,435,000,000.0		43,011,700,000
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent	75.00%		75.00%
Loan to Value Ratio Update Frequency			3 years
Mortgage Loan Inspection Frequency			2 years
Mortage Loan Policy Debt Service Coverage Ratio Lower Range	125.00%		125.00%
Mortgage Loans Issuance, Term Years			25 years
Provision for Loan and Lease Losses	$ 0	$ 600,000	$ 0	$ 800,000
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	$ 0	0
Committments to Fund Commercial Mortgage Loans	$ 16,000,000.0		$ 16,000,000.0
Repurchase agreements - Typical Days Outstanding			P30D
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral	102.00%		102.00%
Net Securities Borrowed Reported in Balance Sheet	$ 153,600,000		$ 153,600,000		164,100,000
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	1,800,000		1,800,000		100,000
Securities Received as Collateral	$ 158,100,000		$ 158,100,000		171,400,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Loss		$ 200,000		200,000	0
Financing Receivable, 90 Days or More Past Due, Count	0		0
Provision for Loan and Lease Losses			$ 100,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Modifications, Number of Contracts \| Integer	0	1	0
Financing Receivable, Troubled Debt Restructuring, Premodification		$ 3,600,000
Impaired Financing Receivable, Average Recorded Investment	$ 0	$ 1,100,000	$ 800,000	$ 1,100,000
Special Purpose Entity \| Bonds
Schedule of Investments
Fixed Maturity Securities	159,900,000		159,900,000		156,700,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	66,500,000		66,500,000		91,500,000
Private Equity Partnerships
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	$ 557,400,000		$ 557,400,000		$ 483,800,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	79		79
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	13		13
Fixed Maturity Securities	$ 44,372,400,000		$ 44,372,400,000
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	52		52
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	33		33
Fixed Maturity Securities	$ 3,062,600,000		$ 3,062,600,000