Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) Integer	Jun. 30, 2018 USD ($) Integer	Jun. 30, 2019 USD ($) Integer	Jun. 30, 2018 USD ($)	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	103,500,000		103,500,000
Carrying Amount of Variable Interest Entity Investments	614,900,000		614,900,000		575,300,000
Unfunded Commitments to Investment Partnerships	2,300,000		2,300,000
Fixed Maturity Securities	$ 46,440,200,000		$ 46,440,200,000		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	$ 200,000	$ 0	$ 200,000
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	$ 87,100,000		$ 87,100,000
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	$ 266,700,000		$ 266,700,000		164,100,000
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	5,800,000		5,800,000		100,000
Securities Received as Collateral	$ 274,500,000		$ 274,500,000		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		1
Provision for Loan and Lease Losses	$ 0	$ 200,000	$ 100,000	200,000
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	$ 1,100,000	$ 1,100,000
Special Purpose Entity | Bonds
Schedule of Investments
Fixed Maturity Securities	159,000,000.0		159,000,000.0		$ 156,700,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	74,600,000		74,600,000		91,500,000
Private Equity Partnerships
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	$ 540,300,000		$ 540,300,000		$ 483,800,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	105		105
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	59		59
Fixed Maturity Securities	$ 43,232,800,000		$ 43,232,800,000
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	51		51
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	41		41
Fixed Maturity Securities	$ 3,207,400,000		$ 3,207,400,000