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 $2.3 million and $2.4 million for the three months ended June 30, 2018 and 2019, respectively, and $6.1 million and $6.5 million for the six months ended June 30, 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 and six months ended June 30, 2018 and 2019 was as follows (in thousands):



	Three Months Ended					Three Months Ended
	June 30, 2018					June 30, 2019
	Pension Benefits			Other Postretirement Benefits		Pension Benefits			Other Postretirement Benefits
Components of net periodic benefit costs:
Service cost	$	6,269		$	51	$	6,358		$	43
Interest cost	2,795			102		3,110			135
Expected return on plan assets	(3,024		)	—		(2,317		)	—
Amortization of prior service credit	(46		)	—		(45		)	—
Amortization of actuarial loss	1,569			134		1,508			117
Settlement cost	—			—		2,060			—
Net periodic benefit cost	$	7,563		$	287	$	10,674		$	295



	Six Months Ended					Six Months Ended
	June 30, 2018					June 30, 2019
	Pension Benefits			Other Postretirement Benefits		Pension Benefits			Other Postretirement Benefits
Components of net periodic benefit costs:
Service cost	$	21,969		$	116	$	12,885		$	97
Interest cost	9,238			208		6,110			254
Expected return on plan assets	(6,002		)	—		(4,691		)	—
Amortization of prior service credit	(91		)	—		(90		)	—
Amortization of actuarial loss	6,523			294		2,785			188
Settlement cost	—			—		2,060			—
Net periodic benefit cost	$	31,637		$	618	$	19,059		$	539

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 (income) 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 and six months ended June 30, 2018 and 2019 were as follows (in thousands):



	Three Months Ended						Three Months Ended
	June 30, 2018						June 30, 2019
Gains (Losses) Included in AOCL	Pension Benefits			Other Postretirement Benefits			Pension Benefits			Other Postretirement Benefits
Beginning balance	$	(98,261	)	$	(6,437	)	$	(87,370	)	$	(5,338	)
Net actuarial gain (loss)	386			267			(10,029		)	(884		)
Amortization of prior service credit	(46		)	—			(45		)	—
Amortization of actuarial loss	1,569			134			1,508			117
Settlement cost	—			—			2,060			—
Ending balance	$	(96,352	)	$	(6,036	)	$	(93,876	)	$	(6,105	)



	Six Months Ended						Six Months Ended
	June 30, 2018						June 30, 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 gain (loss)	(5,558		)	267			(10,029		)	(884		)
Amortization of prior service credit	(91		)	—			(90		)	—
Amortization of actuarial loss	6,523			294			2,785			188
Settlement cost	—			—			2,060			—
Ending balance	$	(96,352	)	$	(6,036	)	$	(93,876	)	$	(6,105	)

Contributions estimated to be paid into the plans in 2019 are $31.6 million and $0.6 million for the pension plans and other postretirement benefit plan, respectively.