Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available For Sale Securities [Abstract]
Less than 12 months, fair value	$ 1,204
Less than 12 months, unrealized losses	(13)
12 months or longer, fair value	617
12 months or longer, unrealized losses	(93)
Total available-for-sale, fair value	1,821
Total available-for-sale, unrealized losses	(106)
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	1,204	1,872
Less than 12 months, unrealized losses	(13)	(62)
12 months or longer, fair value	617	300
12 months or longer, unrealized losses	(93)	(86)
Total available-for-sale, fair value	1,821	2,172
Total available-for-sale, unrealized losses	(106)	(148)
Fixed Maturities [Member] | U S Treasury And Government [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	353	432
Less than 12 months, unrealized losses	(1)	(10)
12 months or longer, fair value	124	0
12 months or longer, unrealized losses	(5)	0
Total available-for-sale, fair value	477	432
Total available-for-sale, unrealized losses	(6)	(10)
Fixed Maturities [Member] | US States And Political Subdivisions [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	203	339
Less than 12 months, unrealized losses	(8)	(13)
12 months or longer, fair value	116	18
12 months or longer, unrealized losses	(4)	(2)
Total available-for-sale, fair value	319	357
Total available-for-sale, unrealized losses	(12)	(15)
Fixed Maturities [Member] | Foreign Government Debt [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	8	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	8	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	425	534
Less than 12 months, unrealized losses	(3)	(29)
12 months or longer, fair value	163	52
12 months or longer, unrealized losses	(77)	(73)
Total available-for-sale, fair value	588	586
Total available-for-sale, unrealized losses	(80)	(102)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	105	436
Less than 12 months, unrealized losses	(1)	(9)
12 months or longer, fair value	156	122
12 months or longer, unrealized losses	(3)	(3)
Total available-for-sale, fair value	261	558
Total available-for-sale, unrealized losses	(4)	(12)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	0	1
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	14	29
12 months or longer, unrealized losses	(4)	(6)
Total available-for-sale, fair value	14	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	27	6
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	5	15
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	32	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	12	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	12	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	71	112
Less than 12 months, unrealized losses	0	(1)
12 months or longer, fair value	39	49
12 months or longer, unrealized losses	0	(2)
Total available-for-sale, fair value	110	161
Total available-for-sale, unrealized losses	$ 0	$ (3)