Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) security	Sep. 30, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 158,000,000	$ 158,000,000
Largest equity holdings to total equity portfolio	3.10%	3.10%
Largest equity holdings to total portfolio	1.00%	1.00%
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 65,000,000	$ 65,000,000	$ 37,000,000
Number Of Securities Other Than Temporary Impairment Losses			20
Fixed maturity investments, Percentage of amortized costs		70.00%	70.00%
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 615,000,000	$ 615,000,000	$ 431,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	4
Provision for Other Credit Losses	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	36	36	69
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 10,000,000	$ 10,000,000	$ 37,000,000
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	1	1	5
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 2,196,000,000	$ 2,196,000,000	$ 1,819,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months	3	3	0
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 55,000,000	$ 55,000,000	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	1	1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value below 70 percent	$ 36,000,000	$ 36,000,000
Largest unrealized investment gains in common stock portfolio [Member]
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 649,000,000	$ 649,000,000
Common equities
Gain (Loss) on Investments [Line Items]
Number of securities	7	7