Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 216,000,000		$ 216,000,000
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)	$ (12,000,000)		$ (12,000,000)		$ (11,000,000)
Fixed maturity investments, Percentage of amortized costs			70.00%		70.00%
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number Of Securities Other Than Temporary Impairment Losses1 | security	0		1
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	94		94		32
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (12,000,000)		$ (12,000,000)		$ (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]
Number Of Securities Other Than Temporary Impairment Losses1 | security	0		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	$ 864,000,000		$ 864,000,000
Common equities
Gain (Loss) on Investments [Line Items]
Number of securities	7		7