(5) Retirement Plans

We sponsor programs that provide retirement benefits to most of our employees. These programs include defined benefit pension plans, defined contribution plans and postretirement healthcare plans. Key terms of our retirement plans are provided in our Annual Report. Our retirement plans costs for the periods ended November 30 were as follows (in millions):

	Three Months Ended				Six Months Ended
	2017		2016		2017		2016
Defined benefit pension plans	$	37	$	58	$	74	$	116
Defined contribution plans		124		112		251		231
Postretirement healthcare plans		18		19		37		38
	$	179	$	189	$	362	$	385

Net periodic benefit cost of the pension and postretirement healthcare plans for the periods ended November 30 included the following components (in millions):

	Three Months Ended
	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2017			2016			2017			2016			2017		2016
Service cost	$	170		$	160		$	23		$	20		$	9	$	9
Interest cost		278			282			13			11			9		10
Expected return on plan assets		(406	)		(375	)		(11	)		(11	)		—		—
Amortization of prior service credit and other		(29	)		(30	)		(1	)		1			—		—
	$	13		$	37		$	24		$	21		$	18	$	19

	Six Months Ended
	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2017			2016			2017			2016			2017		2016
Service cost	$	340		$	319		$	46		$	41		$	18	$	18
Interest cost		557			564			25			22			19		20
Expected return on plan assets		(812	)		(751	)		(22	)		(21	)		—		—
Amortization of prior service credit and other		(59	)		(59	)		(1	)		1			—		—
	$	26		$	73		$	48		$	43		$	37	$	38

Contributions to our tax-qualified U.S. domestic pension plans for the six-month periods ended November 30 were as follows (in millions):

	2017		2016
Required	$	268	$	250
Voluntary		482		250
	$	750	$	500