Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 8,937	$ 9,362
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,949
Percentage of debt securities by credit rating agencies	88.90%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 748
Percentage of debt securities by credit rating agencies	8.40%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 240
Percentage of debt securities by credit rating agencies	2.70%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 88	123
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 88
Percentage of debt securities by credit rating agencies	100.00%
U.S. Treasury bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
U.S. Treasury bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,521	1,649
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,473
Percentage of debt securities by credit rating agencies	96.80%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 47
Percentage of debt securities by credit rating agencies	3.10%
Municipal bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1
Percentage of debt securities by credit rating agencies	0.10%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 223	227
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 211
Percentage of debt securities by credit rating agencies	94.70%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 9
Percentage of debt securities by credit rating agencies	4.00%
Foreign government bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 3
Percentage of debt securities by credit rating agencies	1.30%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 147	177
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 147
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 5,381	5,607
Governmental agency mortgage-backed securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 5,381
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency mortgage-backed securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency mortgage-backed securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,096	1,081
U.S. corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 455
Percentage of debt securities by credit rating agencies	41.50%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 448
Percentage of debt securities by credit rating agencies	40.90%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 193
Percentage of debt securities by credit rating agencies	17.60%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 481	$ 498
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 194
Percentage of debt securities by credit rating agencies	40.30%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 244
Percentage of debt securities by credit rating agencies	50.80%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 43
Percentage of debt securities by credit rating agencies	8.90%