Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) security	Mar. 31, 2016 security	Dec. 31, 2016 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 201,000,000
Largest equity holdings to total equity portfolio	3.70%
Largest equity holdings to total portfolio	1.30%
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (7,000,000)		$ (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	$ 378,000,000		$ 359,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	2
Provision for Other Credit Losses	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	22		32
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (7,000,000)		$ (11,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	0		0
Equity Securities
Gain (Loss) on Investments [Line Items]
Unrealized investment gains due to a net gain position	$ 2,445,000,000		$ 2,348,000,000
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months	1		0
Unrealized losses 12 months or more (equity investment less than $500,000)	$ 0		$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	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	$ 760,000,000
Common equities
Gain (Loss) on Investments [Line Items]
Number of securities	7