Fair Value Measurements - Schedule of Financial Instruments at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets at fair value:
Long-term investment in equity securities		$ 432
U.S. Treasury and Other Foreign Government Securities	$ 734	500
Mutual Funds	16	23
Total assets at fair value	750	955
Level 1
Assets at fair value:
Long-term investment in equity securities		432
U.S. Treasury and Other Foreign Government Securities	734	500
Mutual Funds	16	23
Total assets at fair value	750	955
Level 2
Assets at fair value:
Long-term investment in equity securities		0
U.S. Treasury and Other Foreign Government Securities	0	0
Mutual Funds	0	0
Total assets at fair value	0	0
Level 3
Assets at fair value:
Long-term investment in equity securities		0
U.S. Treasury and Other Foreign Government Securities	0	0
Mutual Funds	0	0
Total assets at fair value	$ 0	$ 0