Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available For Sale Securities [Abstract]
Less than 12 months, fair value	$ 1,972
Less than 12 months, unrealized losses	(52)
12 months or longer, fair value	540
12 months or longer, unrealized losses	(100)
Total available-for-sale, fair value	2,512
Total available-for-sale, unrealized losses	(152)
Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	314
Less than 12 months, unrealized losses	(1)
12 months or longer, fair value	0
12 months or longer, unrealized losses	0
Total held-to-maturity, fair value	314
Total held-to-maturity, unrealized losses	(1)
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	314
Less than 12 months, unrealized losses	(1)
12 months or longer, fair value	0
12 months or longer, unrealized losses	0
Total held-to-maturity, fair value	314
Total held-to-maturity, unrealized losses	(1)
Fixed Maturities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	1,972	$ 1,204
Less than 12 months, unrealized losses	(52)	(13)
12 months or longer, fair value	540	617
12 months or longer, unrealized losses	(100)	(93)
Total available-for-sale, fair value	2,512	1,821
Total available-for-sale, unrealized losses	(152)	(106)
Fixed Maturities [Member] | U S Treasury And Government [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	457	353
Less than 12 months, unrealized losses	(7)	(1)
12 months or longer, fair value	160	124
12 months or longer, unrealized losses	(7)	(5)
Total available-for-sale, fair value	617	477
Total available-for-sale, unrealized losses	(14)	(6)
Fixed Maturities [Member] | US States And Political Subdivisions [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	115	203
Less than 12 months, unrealized losses	(2)	(8)
12 months or longer, fair value	96	116
12 months or longer, unrealized losses	(8)	(4)
Total available-for-sale, fair value	211	319
Total available-for-sale, unrealized losses	(10)	(12)
Fixed Maturities [Member] | Foreign Government Debt [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	8	8
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	0
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	8	8
Total available-for-sale, unrealized losses	0	0
Fixed Maturities [Member] | Corporate Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	1,070	425
Less than 12 months, unrealized losses	(39)	(3)
12 months or longer, fair value	169	163
12 months or longer, unrealized losses	(78)	(77)
Total available-for-sale, fair value	1,239	588
Total available-for-sale, unrealized losses	(117)	(80)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	86	105
Less than 12 months, unrealized losses	(2)	(1)
12 months or longer, fair value	78	156
12 months or longer, unrealized losses	(4)	(3)
Total available-for-sale, fair value	164	261
Total available-for-sale, unrealized losses	(6)	(4)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	5	0
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	15	14
12 months or longer, unrealized losses	(3)	(4)
Total available-for-sale, fair value	20	14
Total available-for-sale, unrealized losses	(3)	(4)
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	45	27
Less than 12 months, unrealized losses	(1)	0
12 months or longer, fair value	1	5
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	46	32
Total available-for-sale, unrealized losses	(1)	0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	39	12
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	0
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	39	12
Total available-for-sale, unrealized losses	0	0
Fixed Maturities [Member] | Other Asset Backed [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	147	71
Less than 12 months, unrealized losses	(1)	0
12 months or longer, fair value	21	39
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	168	110
Total available-for-sale, unrealized losses	$ (1)	$ 0