Investment Components, Including Associated Derivatives (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	$ 145,681		$ 136,160
Unrealized Gains	1,845		3,267
Unrealized Losses	(785)		(423)
Recorded Basis	146,741		139,004
Cash and cash equivalents	12,859	$ 6,884	7,663	$ 8,468	$ 13,928	$ 6,510
Short-term Investments	129,921		125,318
Equity and Other Investments	3,961		6,023
Cash
Cash, Cash Equivalents and Investments [Line Items]
Cash	3,877		3,624
Unrealized Gains	0		0
Unrealized Losses	0		0
Cash and cash equivalents	3,877		3,624
Short-term Investments	0		0
Equity and Other Investments	0		0
Mutual funds
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	1,080		1,478
Unrealized Gains	0		0
Unrealized Losses	0		0
Recorded Basis	1,080		1,478
Cash and cash equivalents	1,080		1,478
Short-term Investments	0		0
Equity and Other Investments	0		0
Commercial paper
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	1,533		319
Unrealized Gains	0		0
Unrealized Losses	0		0
Recorded Basis	1,533		319
Cash and cash equivalents	904		69
Short-term Investments	629		250
Equity and Other Investments	0		0
Certificates of deposit
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	1,832		1,358
Unrealized Gains	0		0
Unrealized Losses	0		0
Recorded Basis	1,832		1,358
Cash and cash equivalents	1,642		972
Short-term Investments	190		386
Equity and Other Investments	0		0
U.S. government and agency securities
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	119,085		112,119
Unrealized Gains	34		85
Unrealized Losses	(746)		(360)
Recorded Basis	118,373		111,844
Cash and cash equivalents	4,465		16
Short-term Investments	113,908		111,828
Equity and Other Investments	0		0
Foreign government bonds
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	6,605		5,276
Unrealized Gains	2		2
Unrealized Losses	(12)		(13)
Recorded Basis	6,595		5,265
Cash and cash equivalents	891		1,504
Short-term Investments	5,704		3,761
Equity and Other Investments	0		0
Mortgage- and asset-backed securities
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	3,952		3,921
Unrealized Gains	7		14
Unrealized Losses	(7)		(4)
Recorded Basis	3,952		3,931
Cash and cash equivalents	0		0
Short-term Investments	3,952		3,931
Equity and Other Investments	0		0
Corporate notes and bonds
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	5,172		4,786
Unrealized Gains	48		61
Unrealized Losses	(14)		(12)
Recorded Basis	5,206		4,835
Cash and cash equivalents	0		0
Short-term Investments	5,206		4,835
Equity and Other Investments	0		0
Municipal securities
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	284		284
Unrealized Gains	47		43
Unrealized Losses	0		0
Recorded Basis	331		327
Cash and cash equivalents	0		0
Short-term Investments	331		327
Equity and Other Investments	0		0
Common and preferred stock
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	1,697		2,472
Unrealized Gains	1,707		3,062
Unrealized Losses	(6)		(34)
Recorded Basis	3,398		5,500
Cash and cash equivalents	0		0
Short-term Investments	0		0
Equity and Other Investments	3,398		5,500
Other investments
Cash, Cash Equivalents and Investments [Line Items]
Cost Basis	564		523
Unrealized Gains	0		0
Unrealized Losses	0		0
Recorded Basis	564		523
Cash and cash equivalents	0		0
Short-term Investments	1		0
Equity and Other Investments	$ 563		$ 523