13.	Employee Benefit Plans and Other Postretirement Benefits

The components of net periodic benefit cost for our pension plans were as follows (dollars in millions):

	Pension Plans
	Three Months Ended March 31,
	2018			2017
Service cost	$	6.2		$	6.1
Interest cost		10.6			10.4
Expected return on plan assets		(14.2	)		(13.5	)
Net amortization of unrecognized amounts
Prior service cost		1.8			1.5
Actuarial loss		2.3			1.9
Net periodic benefit cost	$	6.7		$	6.4

PCA makes pension plan contributions that are sufficient to fund its actuarially determined costs, generally equal to the minimum amounts required by the Employee Retirement Income Security Act (ERISA). From time to time, PCA may make additional discretionary contributions based on the funded status of the plans, tax deductibility, income from operations, and other factors. During the three months ended March 31, 2018 and 2017, payments to our nonqualified pension plans were insignificant. For the three months ended March 31, 2018, we made contributions of $2.2 million to our qualified pension plans. We made contributions of $3.7 million to our qualified plans during the three months ended March 31, 2017. We expect to contribute at least the estimated required minimum contributions to our qualified pension plans of approximately $4.6 million in 2018.

The components of net periodic benefit cost for our postretirement plans were as follows (dollars in millions):

	Postretirement Plans
	Three Months Ended March 31,
	2018			2017
Service cost	$	0.1		$	0.1
Interest cost		0.1			0.1
Net amortization of unrecognized amounts
Prior service cost		(0.1	)		—
Net periodic benefit cost	$	0.1		$	0.2