Investments (Credit Ratings Of Asset-Backed And Mortgage-Backed Securities And Corporate Obligations In Available-For-Sale Investments For Which Fair Value Was Less Than Amortized Cost) (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 1,008	$ 2,802
Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	166
Gross unrealized losses	(7)
Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	280
Gross unrealized losses	(7)
Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	270
Gross unrealized losses	(81)
Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	716
Gross unrealized losses	(95)
Moody's Aaa [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	70
Gross unrealized losses	0
Moody's Aaa [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	231
Gross unrealized losses	(1)
Moody's Aaa [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	86
Gross unrealized losses	(1)
Moody's Aaa [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	387
Gross unrealized losses	(2)
Moodys Aa 1 Rating [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	59
Gross unrealized losses	0
Moodys Aa 1 Rating [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	13
Gross unrealized losses	0
Moodys Aa 1 Rating [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	61
Gross unrealized losses	0
Moodys Aa 1 Rating [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	133
Gross unrealized losses	0
Moody's A1 [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	3
Gross unrealized losses	0
Moody's A1 [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	2
Gross unrealized losses	0
Moody's A1 [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	61
Gross unrealized losses	(1)
Moody's A1 [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	66
Gross unrealized losses	(1)
Moody's Baa 1 Rating [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	2
Gross unrealized losses	0
Moody's Baa 1 Rating [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	1
Gross unrealized losses	0
Moody's Baa 1 Rating [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	29
Gross unrealized losses	(1)
Moody's Baa 1 Rating [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	32
Gross unrealized losses	(1)
Below Investment Grade [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	32
Gross unrealized losses	(7)
Below Investment Grade [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	22
Gross unrealized losses	(3)
Below Investment Grade [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	7
Gross unrealized losses	0
Below Investment Grade [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	61
Gross unrealized losses	(10)
Not Rated [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	0
Gross unrealized losses	0
Not Rated [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	11
Gross unrealized losses	(3)
Not Rated [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	26
Gross unrealized losses	(78)
Not Rated [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	37
Gross unrealized losses	$ (81)