12. Retirement Benefit Plans

We have defined benefit pension plans covering certain U.S. and Puerto Rico employees. The employees who are not participating in the defined benefit plans receive additional benefits under our defined contribution plans. Plan benefits are primarily based on years of credited service and the participant’s compensation. In addition to the U.S. and Puerto Rico defined benefit pension plans, we sponsor various foreign pension arrangements, including retirement and termination benefit plans required by local law or coordinated with government sponsored plans.

The components of net periodic pension expense for our U.S. and foreign defined benefit pension plans are as follows (in millions):

	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2018			2017			2018			2017
Service cost	$	7.0		$	6.4		$	14.5		$	13.9
Interest cost		5.6			4.6			11.1			9.3
Expected return on plan assets		(11.7	)		(10.1	)		(23.5	)		(20.2	)
Curtailment loss		-			0.2			-			0.2
Amortization of prior service cost		(2.5	)		(2.5	)		(5.0	)		(5.1	)
Amortization of unrecognized actuarial loss		6.7			5.2			12.8			10.8
Net periodic pension expense	$	5.1		$	3.8		$	9.9		$	8.9

Service cost is recognized in the operating expense line item in which the related employee is classified. All other components of net periodic pension expense are recognized in Other expense, net.

We expect that we will have minimal legally required funding obligations in 2018 for our U.S. and Puerto Rico defined benefit pension plans, and therefore we have not made, nor do we voluntarily expect to make, any material contributions to these plans during 2018. We contributed $9.8 million to our foreign-based defined benefit pension plans in the six month period ended June 30, 2018, and we expect to contribute $9.7 million to these foreign-based plans during the remainder of 2018.