Fair Value (Financial Assets and Liabilities, Not Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | $ 553 | $ 494 |
Held-to-maturity Securities | 552 | 493 |
Debt and other financing obligations | 35,300 | 34,728 |
Debt and other financing obligations, carrying value | 33,210 | 32,770 |
Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 553 | 494 |
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 275 | 271 |
Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 5 | 8 |
Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 273 | 215 |
U.S. Government and Agency Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities | 259 | 250 |
U.S. Government and Agency Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 260 | 251 |
U.S. Government and Agency Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 257 | 251 |
U.S. Government and Agency Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 3 | 0 |
U.S. Government and Agency Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 0 | 0 |
State and Municipal Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities | 5 | 5 |
State and Municipal Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 5 | 5 |
State and Municipal Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 0 | 0 |
State and Municipal Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 0 | 0 |
State and Municipal Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 5 | 5 |
Corporate Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities | 288 | 238 |
Corporate Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 288 | 238 |
Corporate Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 18 | 20 |
Corporate Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 2 | 8 |
Corporate Obligations [Member] | Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Held-to-maturity Securities, Fair Value | 268 | 210 |
Other Assets [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets | 465 | 471 |
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value Disclosure | 468 | 476 |
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value Disclosure | 0 | 0 |
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value Disclosure | 468 | 476 |
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value Disclosure | 0 | 0 |
Long-term debt and other financing obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt and other financing obligations, carrying value | 30,399 | 29,337 |
Long-term debt and other financing obligations | Fair Value, Measurements, Nonrecurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt and other financing obligations | 32,489 | 31,295 |
Long-term debt and other financing obligations | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets (Level 1) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt and other financing obligations | 0 | 0 |
Long-term debt and other financing obligations | Fair Value, Measurements, Nonrecurring [Member] | Other Observable Inputs (Level 2) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt and other financing obligations | 32,489 | 31,295 |
Long-term debt and other financing obligations | Fair Value, Measurements, Nonrecurring [Member] | Unobservable Inputs (Level 3) [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt and other financing obligations | $ 0 | $ 0 |