Investments - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) security	Sep. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Gain (Loss) on Investments [Line Items]
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0)	$ 75	$ 75	$ 0
Largest equity holdings to total equity portfolio	3.60%	3.60%
Largest equity holdings to total portfolio	1.10%	1.10%
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 10	$ 10	$ 11
Number Of Securities Other Than Temporary Impairment Losses			6
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	$ 493	$ 493	$ 607
Number Of Securities Other Than Temporary Impairment Losses1 | security	2	3
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	73	73	144
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 10	$ 10	$ 10
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 1,613	$ 1,613	$ 2,134
Number Of Securities Other Than Temporary Impairment Losses1 | security	1	3
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months	0	0	3
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 0	$ 0	$ 1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss, Accumulated in Investments			$ 1
Procter And Gamble Company, Exxon Mobile Corporation And Chevron Corporation [Member]
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 494	$ 494
Common equities
Gain (Loss) on Investments [Line Items]
Number of securities	7	7