Marketable Securities and Restricted Funds Schedule of Other Restricted Funds - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Current assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Current	$ 25	$ 11
Investments and other assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Noncurrent	43	38
Cash received for Green Investment Fund | Current assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Current	13
Cash received for Green Investment Fund | Investments and other assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Noncurrent	19
Amounts restricted for insurance purposes | Current assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Current	12	11
Canadian Pipeline Abandonment Requirement | Investments and other assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Noncurrent	17	24
Project Costs | Investments and other assets—other
Gain (Loss) on Investments [Line Items]
Restricted Cash and Cash Equivalents, Noncurrent	$ 7	$ 14