Unrealized Losses on Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Jun. 30, 2017
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 91,112	$ 93,370
Less than 12 Months - Unrealized Losses	(1,130)	(367)
12 Months or Greater - Fair Value	4,877	917
12 Months or Greater - Unrealized Losses	(127)	(56)
Total Fair Value	95,989	94,287
Total Unrealized Losses	(1,257)	(423)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	82,352	87,558
Less than 12 Months - Unrealized Losses	(1,064)	(348)
12 Months or Greater - Fair Value	4,459	371
12 Months or Greater - Unrealized Losses	(103)	(12)
Total Fair Value	86,811	87,929
Total Unrealized Losses	(1,167)	(360)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	3,457	4,006
Less than 12 Months - Unrealized Losses	(7)	(2)
12 Months or Greater - Fair Value	13	23
12 Months or Greater - Unrealized Losses	(3)	(11)
Total Fair Value	3,470	4,029
Total Unrealized Losses	(10)	(13)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	2,072	1,068
Less than 12 Months - Unrealized Losses	(9)	(3)
12 Months or Greater - Fair Value	96	198
12 Months or Greater - Unrealized Losses	(4)	(1)
Total Fair Value	2,168	1,266
Total Unrealized Losses	(13)	(4)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	3,111	669
Less than 12 Months - Unrealized Losses	(43)	(8)
12 Months or Greater - Fair Value	301	177
12 Months or Greater - Unrealized Losses	(13)	(4)
Total Fair Value	3,412	846
Total Unrealized Losses	(56)	(12)
Municipal securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	45
Less than 12 Months - Unrealized Losses	(1)
12 Months or Greater - Fair Value	0
12 Months or Greater - Unrealized Losses	0
Total Fair Value	45
Total Unrealized Losses	(1)
Common and preferred stock
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	75	69
Less than 12 Months - Unrealized Losses	(6)	(6)
12 Months or Greater - Fair Value	8	148
12 Months or Greater - Unrealized Losses	(4)	(28)
Total Fair Value	83	217
Total Unrealized Losses	$ (10)	$ (34)