Reinsurance Reinsurance (Assumed Life Reinsurance Programs Involving Minimum Benefit Guarantees Under Annuity Contracts - Schedule Of Guaranteed Minimum Benefits Income And Expense) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Guaranteed Minimum Benefits [Line Items]
Net premiums earned	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 29,034	$ 28,749	$ 17,213
Policy benefits and other reserve adjustments	176	169	163	168	161	155	146	126	676	588	543
Net realized gains (losses) including OTTI	$ 0	$ (10)	$ 101	$ (7)	$ 365	$ 100	$ (216)	$ (394)	84	(145)	(420)
Guaranteed Minimum Death Benefit
Guaranteed Minimum Benefits [Line Items]
Net premiums earned									49	55	61
Policy benefits and other reserve adjustments									40	45	34
Guaranteed Minimum Income Benefit
Guaranteed Minimum Benefits [Line Items]
Net premiums earned									110	118	121
Policy benefits and other reserve adjustments									105	52	45
Net realized gains (losses) including OTTI									363	48	(203)
Gain (loss) recognized in income									368	114	(127)
Net cash received									65	79	98
Net (increase) decrease in liability									$ 303	$ 35	$ (225)