Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Investments
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0
Commitment to Fund Private Placement Fixed Maturity Securities	213,500,000
Carrying Amount of Variable Interest Entity Investments	532,400,000		$ 535,400,000
Unfunded Commitments to Investment Partnerships	3,700,000
Fair Value of Fixed Maturity Securities	$ 44,500,100,000
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent	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%
Mortgage Loans Issuance, Term Years	25
Number of Financing Receivables on Nonaccrual Status	1		0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 3,200,000
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Impaired Financing Receivable, Average Recorded Investment	$ 1,100,000	$ 0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method	0	0
Committments to Fund Commercial Mortgage Loans	$ 78,900,000
Repurchase agreements - Typical Days Outstanding	30 days
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral	102.00%
Net Securities Borrowed Reported in Balance Sheet	$ 104,200,000		$ 159,200,000
Outstanding Repurchase Agreements	0		0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	16,000,000		30,500,000
Securities Received as Collateral	94,300,000		135,600,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	0		0
Provision for Loan and Lease Losses	$ 0	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 154,400,000		154,100,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	119,100,000		128,200,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	$ 413,300,000		$ 407,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	413
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	141
Fair Value of Fixed Maturity Securities	$ 41,233,800,000
External Credit Rating, Non Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	88
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	20
Fair Value of Fixed Maturity Securities	$ 3,266,300,000