Unrealized Losses on Debt Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Jun. 30, 2018
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 23,053	$ 91,037
Less than 12 Months - Unrealized Losses	(159)	(1,124)
12 Months or Greater - Fair Value	53,986	4,869
12 Months or Greater - Unrealized Losses	(654)	(123)
Total Fair Value	77,039	95,906
Total Unrealized Losses	(813)	(1,247)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	16,776	82,352
Less than 12 Months - Unrealized Losses	(93)	(1,064)
12 Months or Greater - Fair Value	52,757	4,459
12 Months or Greater - Unrealized Losses	(614)	(103)
Total Fair Value	69,533	86,811
Total Unrealized Losses	(707)	(1,167)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	51	3,457
Less than 12 Months - Unrealized Losses	(4)	(7)
12 Months or Greater - Fair Value	87	13
12 Months or Greater - Unrealized Losses	(7)	(3)
Total Fair Value	138	3,470
Total Unrealized Losses	(11)	(10)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	1,825	2,072
Less than 12 Months - Unrealized Losses	(8)	(9)
12 Months or Greater - Fair Value	359	96
12 Months or Greater - Unrealized Losses	(7)	(4)
Total Fair Value	2,184	2,168
Total Unrealized Losses	(15)	(13)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	4,372	3,111
Less than 12 Months - Unrealized Losses	(53)	(43)
12 Months or Greater - Fair Value	770	301
12 Months or Greater - Unrealized Losses	(26)	(13)
Total Fair Value	5,142	3,412
Total Unrealized Losses	(79)	(56)
Municipal securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	29	45
Less than 12 Months - Unrealized Losses	(1)	(1)
12 Months or Greater - Fair Value	13	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	42	45
Total Unrealized Losses	$ (1)	$ (1)