Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available For Sale Securities [Abstract]
Less than 12 months, fair value	$ 1,365
Less than 12 months, unrealized losses	(13)
12 months or longer, fair value	438
12 months or longer, unrealized losses	(85)
Total available-for-sale, fair value	1,803
Total available-for-sale, unrealized losses	(98)
Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	0	$ 0
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	876
12 months or longer, unrealized losses	0	(14)
Total held-to-maturity, fair value	0	876
Total held-to-maturity, unrealized losses	0	(14)
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member]
Held To Maturity Securities [Abstract]
Less than 12 months, fair value	0	0
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	876
12 months or longer, unrealized losses	0	(14)
Total held-to-maturity, fair value	0	876
Total held-to-maturity, unrealized losses	0	(14)
Perpetual Debt And Equity Securities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	0
Less than 12 months, unrealized losses	0
12 months or longer, fair value	0
12 months or longer, unrealized losses	0
Total available-for-sale, fair value	0
Total available-for-sale, unrealized losses	0
Fixed Maturities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	1,365	1,872
Less than 12 months, unrealized losses	(13)	(62)
12 months or longer, fair value	438	300
12 months or longer, unrealized losses	(85)	(86)
Total available-for-sale, fair value	1,803	2,172
Total available-for-sale, unrealized losses	(98)	(148)
Fixed Maturities [Member] | U S Treasury And Government [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	279	432
Less than 12 months, unrealized losses	(2)	(10)
12 months or longer, fair value	87	0
12 months or longer, unrealized losses	(3)	0
Total available-for-sale, fair value	366	432
Total available-for-sale, unrealized losses	(5)	(10)
Fixed Maturities [Member] | US States And Political Subdivisions [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	162	339
Less than 12 months, unrealized losses	(3)	(13)
12 months or longer, fair value	53	18
12 months or longer, unrealized losses	(3)	(2)
Total available-for-sale, fair value	215	357
Total available-for-sale, unrealized losses	(6)	(15)
Fixed Maturities [Member] | Foreign Government Debt [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	4	5
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	4	5
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	388	534
Less than 12 months, unrealized losses	(4)	(29)
12 months or longer, fair value	135	52
12 months or longer, unrealized losses	(71)	(73)
Total available-for-sale, fair value	523	586
Total available-for-sale, unrealized losses	(75)	(102)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	344	436
Less than 12 months, unrealized losses	(4)	(9)
12 months or longer, fair value	140	122
12 months or longer, unrealized losses	(4)	(3)
Total available-for-sale, fair value	484	558
Total available-for-sale, unrealized losses	(8)	(12)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	9	1
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	15	29
12 months or longer, unrealized losses	(4)	(6)
Total available-for-sale, fair value	24	30
Total available-for-sale, unrealized losses	(4)	(6)
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	25	6
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	6	15
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	31	21
Total available-for-sale, unrealized losses	0	0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	5	7
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	15
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	5	22
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	149	112
Less than 12 months, unrealized losses	0	(1)
12 months or longer, fair value	2	49
12 months or longer, unrealized losses	0	(2)
Total available-for-sale, fair value	151	161
Total available-for-sale, unrealized losses	$ 0	$ (3)