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) $ in Millions	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Available For Sale Securities [Abstract]
Fixed-maturity securities held as available-for-sale, amortized cost	$ 3,817	$ 4,438
Available For Sale Securities	$ 3,781	$ 4,423
> 5% To 15% [Member]
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%
> 5% To 15% [Member] | Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	47
Fixed-maturity securities held as available-for-sale, amortized cost	$ 299
Available For Sale Securities	$ 273
> 15% To 25% [Member]
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%
> 15% To 25% [Member] | Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	11
Fixed-maturity securities held as available-for-sale, amortized cost	$ 35
Available For Sale Securities	$ 29
> 25% To 50% [Member]
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%
> 25% To 50% [Member] | Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	3
Fixed-maturity securities held as available-for-sale, amortized cost	$ 12
Available For Sale Securities	$ 9
> 50% [Member]
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	50.00%
> 50% [Member] | Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	3
Fixed-maturity securities held as available-for-sale, amortized cost	$ 99
Available For Sale Securities	$ 30
Greater Than 5% [Member] | Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position | security	64
Fixed-maturity securities held as available-for-sale, amortized cost	$ 445
Available For Sale Securities	$ 341