Employee Benefit Plans

We sponsor a defined contribution plan in which we match our employees’ qualifying contributions, resulting in additional expense to us. Expenses related to the defined contribution plan were $3.8 million and $4.1 million for the three months ended March 31, 2018 and 2019, respectively.

Additionally, we sponsor two union pension plans that cover certain union employees, a pension plan for all non-union employees and a postretirement benefit plan for certain employees. Net periodic benefit expense for the three months ended March 31, 2018 and 2019 was as follows (in thousands):



	Three Months Ended					Three Months Ended
	March 31, 2018					March 31, 2019
	Pension Benefits			Other Postretirement Benefits		Pension Benefits			Other Postretirement Benefits
Components of net periodic benefit costs:
Service cost	$	15,700		$	65	$	6,527		$	54
Interest cost	6,443			106		3,000			119
Expected return on plan assets	(2,978		)	—		(2,374		)	—
Amortization of prior service credit	(45		)	—		(45		)	—
Amortization of actuarial loss	4,954			160		1,277			71
Net periodic benefit cost	$	24,074		$	331	$	8,385		$	244

The service component of our net periodic benefit costs is presented in operating expense and G&A expense, and the non-service components are presented in other expense in our consolidated statements of income.

The changes in accumulated other comprehensive loss (“AOCL”) related to employee benefit plan assets and benefit obligations for the three months ended March 31, 2018 and 2019 were as follows (in thousands):



	Three Months Ended						Three Months Ended
	March 31, 2018						March 31, 2019
Gains (Losses) Included in AOCL	Pension Benefits			Other Postretirement Benefits			Pension Benefits			Other Postretirement Benefits
Beginning balance	$	(97,226	)	$	(6,597	)	$	(88,602	)	$	(5,409	)
Net actuarial loss	(5,944		)	—			—			—
Amortization of prior service credit	(45		)	—			(45		)	—
Amortization of actuarial loss	4,954			160			1,277			71
Ending balance	$	(98,261	)	$	(6,437	)	$	(87,370	)	$	(5,338	)

Contributions estimated to be paid into the plans in 2019 are $32.1 million and $0.2 million for the pension plans and other postretirement benefit plan, respectively.