Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Cash	$ 53	$ 59
Cash equivalents:
Cash and cash equivalents, adjusted cost	753	287
Cash and cash equivalents, fair value disclosure	753	287
U.S. and Canadian government obligations
Cash equivalents:
Cash equivalents, adjusted cost	680	211
Cash equivalents, fair value	680	211
Other debt securities
Cash equivalents:
Cash equivalents, adjusted cost	20	17
Cash equivalents, fair value	20	17
Nonqualified employee benefit trusts
Cash equivalents:
Available-for-sale securities, adjusted cost	16	17
Available-for-sale securities, gross unrealized gain	2	2
Available-for-sale securities, gross unrealized loss	0	0
Available-for-sale securities, fair value	$ 18	$ 19