Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available For Sale Securities [Abstract]
Less than 12 months, fair value	$ 1,955
Less than 12 months, unrealized losses	(51)
12 months or longer, fair value	700
12 months or longer, unrealized losses	(117)
Total available-for-sale, fair value	2,655
Total available-for-sale, unrealized losses	(168)
Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	313
Less than 12 months, unrealized losses	(2)
12 months or longer, fair value	0
12 months or longer, unrealized losses	0
Total held-to-maturity, fair value	313
Total held-to-maturity, unrealized losses	(2)
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	313
Less than 12 months, unrealized losses	(2)
12 months or longer, fair value	0
12 months or longer, unrealized losses	0
Total held-to-maturity, fair value	313
Total held-to-maturity, unrealized losses	(2)
Fixed Maturities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	1,955	$ 1,204
Less than 12 months, unrealized losses	(51)	(13)
12 months or longer, fair value	700	617
12 months or longer, unrealized losses	(117)	(93)
Total available-for-sale, fair value	2,655	1,821
Total available-for-sale, unrealized losses	(168)	(106)
Fixed Maturities [Member] | U S Treasury And Government [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	396	353
Less than 12 months, unrealized losses	(7)	(1)
12 months or longer, fair value	206	124
12 months or longer, unrealized losses	(12)	(5)
Total available-for-sale, fair value	602	477
Total available-for-sale, unrealized losses	(19)	(6)
Fixed Maturities [Member] | US States And Political Subdivisions [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	92	203
Less than 12 months, unrealized losses	(3)	(8)
12 months or longer, fair value	123	116
12 months or longer, unrealized losses	(11)	(4)
Total available-for-sale, fair value	215	319
Total available-for-sale, unrealized losses	(14)	(12)
Fixed Maturities [Member] | Foreign Government Debt [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	9	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	9	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	1,116	425
Less than 12 months, unrealized losses	(38)	(3)
12 months or longer, fair value	213	163
12 months or longer, unrealized losses	(84)	(77)
Total available-for-sale, fair value	1,329	588
Total available-for-sale, unrealized losses	(122)	(80)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	119	105
Less than 12 months, unrealized losses	(2)	(1)
12 months or longer, fair value	95	156
12 months or longer, unrealized losses	(5)	(3)
Total available-for-sale, fair value	214	261
Total available-for-sale, unrealized losses	(7)	(4)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	0	0
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	14	14
12 months or longer, unrealized losses	(3)	(4)
Total available-for-sale, fair value	14	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	29	27
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	19	5
12 months or longer, unrealized losses	(2)	0
Total available-for-sale, fair value	48	32
Total available-for-sale, unrealized losses	(2)	0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	73	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	73	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	121	71
Less than 12 months, unrealized losses	(1)	0
12 months or longer, fair value	30	39
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	151	110
Total available-for-sale, unrealized losses	$ (1)	$ 0