Investments Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | $ 2,426 | $ 2,047 |
Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 2,380 | 1,996 |
Marketable securities | U.S. government and agency debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 337 | 340 |
Marketable securities | Mortgage and asset-backed debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 72 | 74 |
Marketable securities | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 1,483 | 861 |
Marketable securities | U.S. state and local municipal debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 2 | 2 |
Marketable securities | Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 2 | 2 |
Marketable securities | Foreign Government Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 200 | 366 |
Marketable securities | Carbon Credit Investments [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 284 | 351 |
Other Long-term Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 46 | 51 |
Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 640 | 710 |
Fair Value, Inputs, Level 1 | Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 621 | 691 |
Fair Value, Inputs, Level 1 | Marketable securities | U.S. government and agency debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 337 | 340 |
Fair Value, Inputs, Level 1 | Marketable securities | Mortgage and asset-backed debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 1 | Marketable securities | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 1 | Marketable securities | U.S. state and local municipal debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 1 | Marketable securities | Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 1 | Marketable securities | Foreign Government Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 1 | Marketable securities | Carbon Credit Investments [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 284 | 351 |
Fair Value, Inputs, Level 1 | Other Long-term Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 19 | 19 |
Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 1,759 | 1,305 |
Fair Value, Inputs, Level 2 | Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 1,759 | 1,305 |
Fair Value, Inputs, Level 2 | Marketable securities | U.S. government and agency debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 2 | Marketable securities | Mortgage and asset-backed debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 72 | 74 |
Fair Value, Inputs, Level 2 | Marketable securities | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 1,483 | 861 |
Fair Value, Inputs, Level 2 | Marketable securities | U.S. state and local municipal debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 2 | 2 |
Fair Value, Inputs, Level 2 | Marketable securities | Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 2 | 2 |
Fair Value, Inputs, Level 2 | Marketable securities | Foreign Government Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 200 | 366 |
Fair Value, Inputs, Level 2 | Marketable securities | Carbon Credit Investments [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 2 | Other Long-term Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 27 | 32 |
Fair Value, Inputs, Level 3 | Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | U.S. government and agency debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | Mortgage and asset-backed debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | U.S. state and local municipal debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | Foreign Government Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Marketable securities | Carbon Credit Investments [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | 0 | 0 |
Fair Value, Inputs, Level 3 | Other Long-term Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Investments | $ 27 | $ 32 |