Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost		$ 2,452
Available-for-sale debt securities, Unrealized gains		2
Available-for-sale debt securities, Unrealized losses		(27)
Available-for-sale debt securities, Fair value		2,427
Equity investments, Amortized cost	$ 2,196
Equity investments, Unrealized gains	11
Equity investments, Unrealized losses	(23)
Equity investments, Fair value	2,184	2,176
Total investments in marketable securities, Amortized cost	5,865
Total investments in marketable securities, Unrealized gain	15
Total investments in marketable securities, Unrealized losses	(51)
Investments in marketable securities	5,829	4,603
Equity investments	2,184	2,176
U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	2,821	2,150
Available-for-sale debt securities, Unrealized gains	0	0
Available-for-sale debt securities, Unrealized losses	(23)	(20)
Available-for-sale debt securities, Fair value	2,798	2,130
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	10	9
Available-for-sale debt securities, Unrealized gains	1	1
Available-for-sale debt securities, Unrealized losses	0	0
Available-for-sale debt securities, Fair value	11	10
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	475
Available-for-sale debt securities, Unrealized gains	0
Available-for-sale debt securities, Unrealized losses	(1)
Available-for-sale debt securities, Fair value	474
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	52
Available-for-sale debt securities, Unrealized gains	0
Available-for-sale debt securities, Unrealized losses	0
Available-for-sale debt securities, Fair value	52
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	203	180
Available-for-sale debt securities, Unrealized gains	1	0
Available-for-sale debt securities, Unrealized losses	(3)	(4)
Available-for-sale debt securities, Fair value	201	176
U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	38	36
Available-for-sale debt securities, Unrealized gains	1	0
Available-for-sale debt securities, Unrealized losses	0	(1)
Available-for-sale debt securities, Fair value	39	35
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	1	1
Available-for-sale debt securities, Unrealized gains	1	1
Available-for-sale debt securities, Unrealized losses	0	0
Available-for-sale debt securities, Fair value	2	2
Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	30	29
Available-for-sale debt securities, Unrealized gains	0	0
Available-for-sale debt securities, Unrealized losses	(1)	(1)
Available-for-sale debt securities, Fair value	29	28
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, Amortized cost	39	47
Available-for-sale debt securities, Unrealized gains	0	0
Available-for-sale debt securities, Unrealized losses	0	(1)
Available-for-sale debt securities, Fair value	$ 39	$ 46

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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