Unrealized Losses on Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 104,462	$ 93,370
Less than 12 Months - Unrealized Losses	(297)	(367)
12 Months or Greater - Fair Value	5,559	917
12 Months or Greater - Unrealized Losses	(81)	(56)
Total Fair Value	110,021	94,287
Total Unrealized Losses	(378)	(423)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	98,245	87,558
Less than 12 Months - Unrealized Losses	(290)	(348)
12 Months or Greater - Fair Value	4,957	371
12 Months or Greater - Unrealized Losses	(54)	(12)
Total Fair Value	103,202	87,929
Total Unrealized Losses	(344)	(360)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	4,972	4,006
Less than 12 Months - Unrealized Losses	(1)	(2)
12 Months or Greater - Fair Value	24	23
12 Months or Greater - Unrealized Losses	(11)	(11)
Total Fair Value	4,996	4,029
Total Unrealized Losses	(12)	(13)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	713	1,068
Less than 12 Months - Unrealized Losses	(2)	(3)
12 Months or Greater - Fair Value	236	198
12 Months or Greater - Unrealized Losses	(2)	(1)
Total Fair Value	949	1,266
Total Unrealized Losses	(4)	(4)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	502	669
Less than 12 Months - Unrealized Losses	(4)	(8)
12 Months or Greater - Fair Value	283	177
12 Months or Greater - Unrealized Losses	(5)	(4)
Total Fair Value	785	846
Total Unrealized Losses	(9)	(12)
Common and preferred stock
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	30	69
Less than 12 Months - Unrealized Losses	0	(6)
12 Months or Greater - Fair Value	59	148
12 Months or Greater - Unrealized Losses	(9)	(28)
Total Fair Value	89	217
Total Unrealized Losses	$ (9)	$ (34)