Investments (Average Credit Ratings For Securities Rated Below Investment Grade Without The Effect Of Financial Guarantees) (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Asset-backed [Member]
Schedule Of Investments [Line Items]
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	$ 32
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	65.00%
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	$ 18
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	100.00%
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	$ 0
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors	0.00%