Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Integer	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Integer	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
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	146,700,000		146,700,000
Carrying Amount of Variable Interest Entity Investments	556,200,000		556,200,000			535,400,000
Unfunded Commitments to Investment Partnerships	3,700,000		3,700,000
Fixed Maturity Securities	$ 43,360,900,000		$ 43,360,900,000			45,457,800,000
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
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Provision for Loan and Lease Losses	$ 200,000	$ 0	$ 200,000	$ 0
Impaired Financing Receivable, Interest Income, Accrual Method	0	0	0	0
Committments to Fund Commercial Mortgage Loans	$ 75,000,000		$ 75,000,000
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%		102.00%
Net Securities Borrowed Reported in Balance Sheet	$ 59,700,000		$ 59,700,000			159,200,000
Outstanding Repurchase Agreements	0		0			0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	14,900,000		14,900,000			30,500,000
Securities Received as Collateral	47,300,000		47,300,000			135,600,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	200,000		200,000			0
Number of Financing Receivables on Nonaccrual Status	0		0			0
Loans and Leases Receivable, Allowance	200,000	0	200,000	0	$ 0	0	$ 0	$ 0
Provision for Loan and Lease Losses	$ 200,000	$ 0	$ 200,000	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Modifications, Number of Contracts	1	0	1	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment			$ 3,600,000
Impaired Financing Receivable, Recorded Investment	$ 3,400,000		3,400,000
Impaired Financing Receivable, Average Recorded Investment	1,100,000	$ 0	1,100,000	$ 0
Special Purpose Entity | Bonds
Schedule of Investments
Fixed Maturity Securities	154,900,000		154,900,000			154,100,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	110,000,000		110,000,000			128,200,000
Private Equity Partnerships
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	$ 446,200,000		$ 446,200,000			$ 407,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	499		499
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	137		137
Fixed Maturity Securities	$ 40,157,100,000		$ 40,157,100,000
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	104		104
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	20		20
Fixed Maturity Securities	$ 3,203,800,000		$ 3,203,800,000