Investments - Additional Information (Detail)	3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Integer	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 45,422,600,000			$ 45,422,600,000		$ 44,217,300,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0			0		0
Commitment to Fund Private Placement Fixed Maturity Securities	133,400,000			133,400,000
Carrying Amount of Variable Interest Entity Investments	543,400,000			543,400,000		509,300,000
Unfunded Commitments to Investment Partnerships	4,400,000			4,400,000
Unfunded Unconditional Commitment to Fund Private Equity Partnerships	100,000			100,000
Commitment to Fund Partnership Equity Investments	366,100,000			366,100,000
Other Long-term Investments	654,500,000			654,500,000		$ 631,500,000
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	$ (9,800,000)		$ (11,000,000)	$ (28,900,000)	$ (26,300,000)
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
Impaired Financing Receivable, Recorded Investment			5,400,000		5,400,000
Number of Financing Receivables on Nonaccrual Status	0			0		0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans				0
Impaired Financing Receivable, Average Recorded Investment	$ 0		3,600,000	$ 0	3,000,000
Impaired Financing Receivable, Interest Income, Accrual Method	0		0	0	0
Committments to Fund Commercial Mortgage Loans	$ 93,000,000			$ 93,000,000
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	$ 169,900,000			$ 169,900,000		$ 178,500,000
Outstanding Repurchase Agreements	0			0		0
Federal Home Loan Bank Stock	34,200,000			34,200,000		31,600,000
Federal Home Loan Bank (FHLB) Funding Agreements	350,000,000			350,000,000		350,000,000
Fixed Maturity Securities
Schedule of Investments
Fair Value of Fixed Maturity Securities	45,422,600,000			45,422,600,000		44,217,300,000
Accumulated Gross Unrealized Gain on Securities	5,698,300,000			5,698,300,000		4,883,200,000
Accumulated Gross Unrealized Loss on Securities	110,600,000			110,600,000		218,600,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	207,100,000			207,100,000		323,700,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	343,800,000			343,800,000		288,500,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	32,300,000			32,300,000		29,900,000
Securities Received as Collateral	146,100,000			146,100,000		155,300,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	0			0		0
Provision for Loan and Lease Losses	$ 0		$ 0	$ 0	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0		1	0	1
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 154,200,000			$ 154,200,000		151,900,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments						1,000,000
Gain (Loss) on Investments, Excluding Other than Temporary Impairments		$ 200,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	137,400,000			137,400,000		165,200,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	406,000,000			406,000,000		$ 344,100,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	42,034,700,000			42,034,700,000
Accumulated Gross Unrealized Gain on Securities	5,506,700,000			5,506,700,000
Accumulated Gross Unrealized Loss on Securities	$ 52,700,000			$ 52,700,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	47.60%			47.60%
Number of Fixed Maturity Securities in an Unrealized Loss Position	206			206
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	109			109
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,387,900,000			$ 3,387,900,000
Accumulated Gross Unrealized Gain on Securities	191,600,000			191,600,000
Accumulated Gross Unrealized Loss on Securities	$ 57,900,000			$ 57,900,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	52.40%			52.40%
Number of Fixed Maturity Securities in an Unrealized Loss Position	37			37
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	27			27
Starmount
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 38,700,000			$ 38,700,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	$ (500,000)			$ (500,000)
Starmount | External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	106			106