Unrealized Losses on Debt Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 4,196	$ 4,552
Less than 12 Months - Unrealized Losses	(22)	(31)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	4,196	4,552
Total Unrealized Losses	(22)	(31)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	1,847	2,323
Less than 12 Months - Unrealized Losses	0	(1)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	1,847	2,323
Total Unrealized Losses	0	(1)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	977	500
Less than 12 Months - Unrealized Losses	(3)	(3)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	977	500
Total Unrealized Losses	(3)	(3)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	832	1,014
Less than 12 Months - Unrealized Losses	(4)	(6)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	832	1,014
Total Unrealized Losses	(4)	(6)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	421	649
Less than 12 Months - Unrealized Losses	(5)	(17)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	421	649
Total Unrealized Losses	(5)	(17)
Municipal securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	119	66
Less than 12 Months - Unrealized Losses	(10)	(4)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	119	66
Total Unrealized Losses	$ (10)	$ (4)