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Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Detail)
$ in Millions
Dec. 31, 2021
USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost $ 10,135
Market value 10,628
Amount at which shown in the balance sheet 10,628
Short-term Investments  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 58
Market value 58
Amount at which shown in the balance sheet 58
U.S. Treasury bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 123
Market value 123
Amount at which shown in the balance sheet 123
Municipal bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 1,607
Market value 1,649
Amount at which shown in the balance sheet 1,649
Foreign government bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 228
Market value 227
Amount at which shown in the balance sheet 227
Governmental agency bonds  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 175
Market value 177
Amount at which shown in the balance sheet 177
Governmental agency mortgage-backed securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 5,620
Market value 5,607
Amount at which shown in the balance sheet 5,607
U.S. corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 1,071
Market value 1,081
Amount at which shown in the balance sheet 1,081
Foreign corporate debt securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 493
Market value 498
Amount at which shown in the balance sheet 498
Debt Securities  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 9,317
Market value 9,362
Amount at which shown in the balance sheet 9,362
Equity Securities, Investment Summary  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 728
Market value 1,176 [1]
Amount at which shown in the balance sheet 1,176
Notes Receivable  
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]  
Cost 32
Market value 32
Amount at which shown in the balance sheet $ 32
[1] Included in equity securities are non-marketable equity securities and equity method investments.  Estimates of fair value for these investments could not be made without incurring excessive costs.