Income Taxes - Provision for Federal & Foreign Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current
Operations	$ (189)	$ 1,327	$ 1,573
One-time charge due to tax legislation	43	0	0
Deferred
Operations	1,613	(231)	(473)
One-time charge due to tax legislation	1,819	0	0
Total federal income tax expense	3,286	1,096	1,100
Foreign income tax expense (benefit):
Current	53	56	39
Deferred	1	(19)	34
Total foreign income tax expense	54	37	73
Total income tax expense	$ 3,340	$ 1,133	$ 1,173