Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 206,000,000		$ 206,000,000
Largest equity holdings to total equity portfolio	3.70%		3.70%
Largest equity holdings to total portfolio	1.30%		1.30%
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (6,000,000)		$ (6,000,000)		$ (11,000,000)
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	$ 431,000,000		$ 431,000,000		$ 359,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	1		1
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	23		23		32
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (6,000,000)		$ (6,000,000)		$ (11,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	0		0		0
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 2,597,000,000		$ 2,597,000,000		$ 2,348,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	5		5
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months	0		0		0
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 0		$ 0		$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	0		0		0
Largest unrealized investment gains in common stock portfolio [Member]
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 829,000,000		$ 829,000,000
Common equities
Gain (Loss) on Investments [Line Items]
Number of securities	7		7