Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 7,210	$ 5,772
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,436
Percentage of debt securities in unrealized loss position	89.30%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 558
Percentage of debt securities in unrealized loss position	7.70%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 216
Percentage of debt securities in unrealized loss position	3.00%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 87	76
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 87
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
U.S. Treasury bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,024	684
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 988
Percentage of debt securities in unrealized loss position	96.50%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 35
Percentage of debt securities in unrealized loss position	3.40%
Municipal bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1
Percentage of debt securities in unrealized loss position	0.10%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 213	136
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 204
Percentage of debt securities in unrealized loss position	95.80%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 7
Percentage of debt securities in unrealized loss position	3.30%
Foreign government bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2
Percentage of debt securities in unrealized loss position	0.90%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 101	73
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 101
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental agency bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,540	4,036
Governmental agency mortgage-backed securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,540
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency mortgage-backed securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental agency mortgage-backed securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 851	533
U.S. corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 344
Percentage of debt securities in unrealized loss position	40.40%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 334
Percentage of debt securities in unrealized loss position	39.30%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 173
Percentage of debt securities in unrealized loss position	20.30%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 394	$ 234
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 172
Percentage of debt securities in unrealized loss position	43.60%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 182
Percentage of debt securities in unrealized loss position	46.20%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 40
Percentage of debt securities in unrealized loss position	10.20%