Fair Value Measurements (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment
Unrealized loss on embedded derivative	$ 2	$ 4
Cash	54		$ 120
Cash equivalents:
Cash and cash equivalents, adjusted cost	1,022		835
Cash and cash equivalents, fair value disclosure	1,022		835
Nonqualified employee benefit trusts
Cash equivalents:
Available-for-sale securities, adjusted cost	18		17
Available-for-sale securities, gross unrealized gain	2		2
Available-for-sale securities, fair value	20		19
Other debt securities
Cash equivalents:
Cash equivalents, adjusted cost	5		5
Cash equivalents, fair value	5		5
U.S. and Canadian government obligations
Cash equivalents:
Cash equivalents, adjusted cost	963		710
Cash equivalents, fair value	$ 963		$ 710