Unrealized Losses on Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Jun. 30, 2017
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	$ 106,541	$ 93,370
Less than 12 Months - Unrealized Losses	(682)	(367)
12 Months or Greater - Fair Value	3,702	917
12 Months or Greater - Unrealized Losses	(103)	(56)
Total Fair Value	110,243	94,287
Total Unrealized Losses	(785)	(423)
U.S. government and agency securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	101,552	87,558
Less than 12 Months - Unrealized Losses	(672)	(348)
12 Months or Greater - Fair Value	2,855	371
12 Months or Greater - Unrealized Losses	(74)	(12)
Total Fair Value	104,407	87,929
Total Unrealized Losses	(746)	(360)
Foreign government bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	2,537	4,006
Less than 12 Months - Unrealized Losses	(1)	(2)
12 Months or Greater - Fair Value	34	23
12 Months or Greater - Unrealized Losses	(11)	(11)
Total Fair Value	2,571	4,029
Total Unrealized Losses	(12)	(13)
Mortgage- and asset-backed securities
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	1,317	1,068
Less than 12 Months - Unrealized Losses	(3)	(3)
12 Months or Greater - Fair Value	338	198
12 Months or Greater - Unrealized Losses	(4)	(1)
Total Fair Value	1,655	1,266
Total Unrealized Losses	(7)	(4)
Corporate notes and bonds
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	1,118	669
Less than 12 Months - Unrealized Losses	(6)	(8)
12 Months or Greater - Fair Value	439	177
12 Months or Greater - Unrealized Losses	(8)	(4)
Total Fair Value	1,557	846
Total Unrealized Losses	(14)	(12)
Common and preferred stock
Unrealized Losses on Investments [Line Items]
Less than 12 Months - Fair Value	17	69
Less than 12 Months - Unrealized Losses	0	(6)
12 Months or Greater - Fair Value	36	148
12 Months or Greater - Unrealized Losses	(6)	(28)
Total Fair Value	53	217
Total Unrealized Losses	$ (6)	$ (34)