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Fair Value (Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 19,832 $ 16,921
Debt Securities, Available-for-sale 42,125 39,828
Equity Securities, Fair Value 1,937 1,828
Assets under management 4,198 4,076
Assets, Fair Value Disclosure 68,092 62,653
Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 42,125 39,828
U.S. Government and Agency Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 3,597 3,465
State and Municipal Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 7,423 7,328
Corporate Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 21,687 19,737
U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 6,733 7,091
Non-U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 2,685 2,207
Quoted Prices in Active Markets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 19,703 16,841
Equity Securities, Fair Value 1,917 1,795
Assets under management 1,803 1,774
Assets, Fair Value Disclosure 26,842 23,676
Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 3,419 3,266
Quoted Prices in Active Markets (Level 1) [Member] | U.S. Government and Agency Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 3,379 3,241
Quoted Prices in Active Markets (Level 1) [Member] | State and Municipal Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Quoted Prices in Active Markets (Level 1) [Member] | Corporate Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 40 25
Quoted Prices in Active Markets (Level 1) [Member] | U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Quoted Prices in Active Markets (Level 1) [Member] | Non-U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 129 80
Equity Securities, Fair Value 20 33
Assets under management 2,292 2,250
Assets, Fair Value Disclosure 40,916 38,637
Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 38,475 36,274
Other Observable Inputs (Level 2) [Member] | U.S. Government and Agency Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 218 224
Other Observable Inputs (Level 2) [Member] | State and Municipal Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 7,423 7,328
Other Observable Inputs (Level 2) [Member] | Corporate Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 21,416 19,424
Other Observable Inputs (Level 2) [Member] | U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 6,733 7,091
Other Observable Inputs (Level 2) [Member] | Non-U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 2,685 2,207
Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Equity Securities, Fair Value 0 0
Assets under management 103 52
Assets, Fair Value Disclosure 334 340
Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 231 288
Unobservable Inputs (Level 3) [Member] | U.S. Government and Agency Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Unobservable Inputs (Level 3) [Member] | State and Municipal Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Unobservable Inputs (Level 3) [Member] | Corporate Obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 231 288
Unobservable Inputs (Level 3) [Member] | U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale 0 0
Unobservable Inputs (Level 3) [Member] | Non-U.S. Agency Mortgage-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Available-for-sale $ 0 $ 0