Investments - Additional Information (Detail)	3 Months Ended			9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 46,315,500,000			$ 46,315,500,000			$ 43,354,400,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0			0			0
Commitment to Fund Private Placement Fixed Maturity Securities	75,500,000			75,500,000
Carrying Amount of Variable Interest Entity Investments	507,900,000			507,900,000
Unfunded Commitments to Investment Partnerships	4,800,000			4,800,000
Unfunded Unconditional Commitment to Fund Private Equity Partnerships	600,000			600,000
Commitment to Fund Partnership Equity Investments	260,400,000			260,400,000
Other Long-term Investments	634,300,000			634,300,000			$ 583,000,000
Commitments to Fund Special Purpose Entities	$ 0			$ 0
Amount Funded to Special Purpose Entity Partnerships				nor did we fund any amounts to the partnership	nor did we fund any amounts to the partnership
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
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
Average Recorded Investment in Impaired Mortgage Loans	$ 3,600,000	$ 8,400,000		$ 3,000,000	$ 11,500,000
Interest Income on Mortgage Loans Subsequent to Impairment		200,000			$ 600,000
Committments to Fund Commercial Mortgage Loans	$ 38,000,000			$ 38,000,000
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	$ 210,000,000			$ 210,000,000			$ 181,600,000
Outstanding Repurchase Agreements	0			0			0
Federal Home Loan Bank Stock	31,600,000			31,600,000			30,900,000
Federal Home Loan Bank (FHLB) Funding Agreements	349,500,000			349,500,000			350,000,000
Fixed Maturity Securities
Schedule of Investments
Fair Value of Fixed Maturity Securities	46,315,500,000			46,315,500,000			43,354,400,000
Accumulated Gross Unrealized Gain on Securities	6,774,500,000			6,774,500,000			4,338,000,000
Accumulated Gross Unrealized Loss on Securities	84,100,000			84,100,000			642,300,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	329,200,000			329,200,000			317,200,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	81,100,000			81,100,000			96,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	56,600,000			56,600,000			29,000,000
Securities Received as Collateral	161,300,000			161,300,000			159,300,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	$ 0			0		$ 2,000,000	$ 0
Financing Receivable, Allowance for Credit Losses, Write-downs		2,000,000
Impaired Financing Receivable, Related Allowance								$ 1,500,000
Provision for Loan and Lease Losses			$ 500,000	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	1	0		1	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 153,700,000			$ 153,700,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	900,000			900,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	174,400,000			174,400,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	333,500,000			333,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	42,704,100,000			42,704,100,000
Accumulated Gross Unrealized Gain on Securities	6,608,900,000			6,608,900,000
Accumulated Gross Unrealized Loss on Securities	$ 15,700,000			$ 15,700,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	18.70%			18.70%
Number of Fixed Maturity Securities in an Unrealized Loss Position	147			147
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	17			17
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,611,400,000			$ 3,611,400,000
Accumulated Gross Unrealized Gain on Securities	165,600,000			165,600,000
Accumulated Gross Unrealized Loss on Securities	$ 68,400,000			$ 68,400,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	81.30%			81.30%
Number of Fixed Maturity Securities in an Unrealized Loss Position	55			55
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	43			43
Starmount
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 39,300,000			$ 39,300,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 200,000			$ 200,000
Starmount | External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	113			113