Condensed Financial Statements - Other Investments (Details) - BYD Company Limited common stock [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments, Including Equity Method And Restricted Cash And Investments [Line Items]
Available-for-sale Securities, Equity Securities	$ 1,961	$ 1,185
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes	1,729	953
Parent Company [Member]
Investments, Including Equity Method And Restricted Cash And Investments [Line Items]
Available-for-sale Securities, Equity Securities	1,961	1,185
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes	$ 1,729	$ 953