Investments (Distribution Of Securities By Percentage Of Fair Value Below Book Value By More Than 5% For A Continuous Twelve Month Period Or Longer) (Detail) - Unrealized loss position > 12 months $ in Millions	Sep. 30, 2020 USD ($) security
> 5% To 15% [Member]
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	10
Fixed-maturity securities held as available-for-sale, amortized cost	$ 20
Available For Sale Securities	$ 18
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	5.00%
Percentage Of Fair Value Below Book Value Maximum	15.00%
> 15% To 25% [Member]
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	2
Fixed-maturity securities held as available-for-sale, amortized cost	$ 0
Available For Sale Securities	$ 0
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	15.00%
Percentage Of Fair Value Below Book Value Maximum	25.00%
> 25% To 50% [Member]
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	2
Fixed-maturity securities held as available-for-sale, amortized cost	$ 1
Available For Sale Securities	$ 1
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	25.00%
Percentage Of Fair Value Below Book Value Maximum	50.00%
> 50% [Member]
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	2
Fixed-maturity securities held as available-for-sale, amortized cost	$ 0
Available For Sale Securities	$ 0
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	50.00%
Greater Than 5% [Member]
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	16
Fixed-maturity securities held as available-for-sale, amortized cost	$ 21
Available For Sale Securities	$ 19