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	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)
Available For Sale Securities [Abstract]
Fixed-maturity securities held as available-for-sale, amortized cost	$ 3,734	$ 3,817
Available For Sale Securities	$ 3,776	$ 3,781
> 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	41
Fixed-maturity securities held as available-for-sale, amortized cost	$ 212
Available For Sale Securities	$ 195
> 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	5
Fixed-maturity securities held as available-for-sale, amortized cost	$ 2
Available For Sale Securities	$ 2
> 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	2
Fixed-maturity securities held as available-for-sale, amortized cost	$ 14
Available For Sale Securities	$ 10
> 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	$ 71
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	51
Fixed-maturity securities held as available-for-sale, amortized cost	$ 299
Available For Sale Securities	$ 237