Unrealized Losses on Debt Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 72,379	$ 91,037
Less than 12 Months - Unrealized Losses	(1,014)	(1,124)
12 Months or Greater - Fair Value	28,245	4,869
12 Months or Greater - Unrealized Losses	(508)	(123)
Total Fair Value	100,624	95,906
Total Unrealized Losses	(1,522)	(1,247)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	65,155	82,352
Less than 12 Months - Unrealized Losses	(960)	(1,064)
12 Months or Greater - Fair Value	27,736	4,459
12 Months or Greater - Unrealized Losses	(482)	(103)
Total Fair Value	92,891	86,811
Total Unrealized Losses	(1,442)	(1,167)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	3,160	3,457
Less than 12 Months - Unrealized Losses	(8)	(7)
12 Months or Greater - Fair Value	26	13
12 Months or Greater - Unrealized Losses	(4)	(3)
Total Fair Value	3,186	3,470
Total Unrealized Losses	(12)	(10)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	1,130	2,072
Less than 12 Months - Unrealized Losses	(8)	(9)
12 Months or Greater - Fair Value	145	96
12 Months or Greater - Unrealized Losses	(6)	(4)
Total Fair Value	1,275	2,168
Total Unrealized Losses	(14)	(13)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	2,892	3,111
Less than 12 Months - Unrealized Losses	(37)	(43)
12 Months or Greater - Fair Value	338	301
12 Months or Greater - Unrealized Losses	(16)	(13)
Total Fair Value	3,230	3,412
Total Unrealized Losses	(53)	(56)
Municipal securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	42	45
Less than 12 Months - Unrealized Losses	(1)	(1)
12 Months or Greater - Fair Value	0	0
12 Months or Greater - Unrealized Losses	0	0
Total Fair Value	42	45
Total Unrealized Losses	$ (1)	$ (1)