Marketable Securities (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | $ 1,123 | $ 1,252 |
Cash and cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 93 | 114 |
Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 56 | 56 |
Prepaids and other assets | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 100 | 90 |
Other assets | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 874 | 992 |
Level One - Quoted Prices in Active Markets for Identical Assets | Interest bearing money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 93 | 114 |
Level One - Quoted Prices in Active Markets for Identical Assets | Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 388 | 352 |
Level One - Quoted Prices in Active Markets for Identical Assets | Common shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 127 | |
Level One - Quoted Prices in Active Markets for Identical Assets | Cash and cash equivalents | Interest bearing money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 93 | 114 |
Level One - Quoted Prices in Active Markets for Identical Assets | Cash and cash equivalents | Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Cash and cash equivalents | Common shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | |
Level One - Quoted Prices in Active Markets for Identical Assets | Short-term investments | Interest bearing money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Short-term investments | Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Short-term investments | Common shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | |
Level One - Quoted Prices in Active Markets for Identical Assets | Prepaids and other assets | Interest bearing money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Prepaids and other assets | Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Prepaids and other assets | Common shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | |
Level One - Quoted Prices in Active Markets for Identical Assets | Other assets | Interest bearing money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level One - Quoted Prices in Active Markets for Identical Assets | Other assets | Mutual funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 388 | 352 |
Level One - Quoted Prices in Active Markets for Identical Assets | Other assets | Common shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 127 | |
Level Two - Significant Other Observable Inputs | Time deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 60 | 59 |
Level Two - Significant Other Observable Inputs | U.S. government obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 155 | 142 |
Level Two - Significant Other Observable Inputs | U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 50 | 53 |
Level Two - Significant Other Observable Inputs | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 184 | 181 |
Level Two - Significant Other Observable Inputs | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 21 | 22 |
Level Two - Significant Other Observable Inputs | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 42 | 34 |
Level Two - Significant Other Observable Inputs | Municipal and provincial notes and bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 3 | 5 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | Time deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | U.S. government obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Cash and cash equivalents | Municipal and provincial notes and bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Short-term investments | Time deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 49 | 46 |
Level Two - Significant Other Observable Inputs | Short-term investments | U.S. government obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Short-term investments | U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 3 | 9 |
Level Two - Significant Other Observable Inputs | Short-term investments | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 4 | 1 |
Level Two - Significant Other Observable Inputs | Short-term investments | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Short-term investments | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Short-term investments | Municipal and provincial notes and bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | Time deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 0 | 0 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | U.S. government obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 39 | 33 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 8 | 8 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 37 | 35 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 5 | 5 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 10 | 8 |
Level Two - Significant Other Observable Inputs | Prepaids and other assets | Municipal and provincial notes and bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 1 | 1 |
Level Two - Significant Other Observable Inputs | Other assets | Time deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 11 | 13 |
Level Two - Significant Other Observable Inputs | Other assets | U.S. government obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 116 | 109 |
Level Two - Significant Other Observable Inputs | Other assets | U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 39 | 36 |
Level Two - Significant Other Observable Inputs | Other assets | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 143 | 145 |
Level Two - Significant Other Observable Inputs | Other assets | Mortgage-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 16 | 17 |
Level Two - Significant Other Observable Inputs | Other assets | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | 32 | 26 |
Level Two - Significant Other Observable Inputs | Other assets | Municipal and provincial notes and bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | $ 2 | 4 |
Level Three - Significant Unobservable Inputs | Preferred shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | | 290 |
Level Three - Significant Unobservable Inputs | Cash and cash equivalents | Preferred shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | | 0 |
Level Three - Significant Unobservable Inputs | Short-term investments | Preferred shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | | 0 |
Level Three - Significant Unobservable Inputs | Prepaids and other assets | Preferred shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | | 0 |
Level Three - Significant Unobservable Inputs | Other assets | Preferred shares | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis | | |
Investments, fair value disclosure | | $ 290 |