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 $3.3 million and $3.8 million for the three months ended March 31, 2017 and 2018, 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, 2017 and 2018 was as follows (in thousands):



	Three Months Ended					Three Months Ended
	March 31, 2017					March 31, 2018
	Pension Benefits			Other Postretirement Benefits		Pension Benefits			Other Postretirement Benefits
Components of net periodic benefit costs:
Service cost	$	5,018		$	65	$	15,700		$	65
Interest cost	2,350			122		6,443			106
Expected return on plan assets	(2,487		)	—		(2,978		)	—
Amortization of prior service credit	(45		)	—		(45		)	—
Amortization of actuarial loss	1,028			200		4,954			160
Settlement cost	1,365			—		—			—
Net periodic benefit cost	$	7,229		$	387	$	24,074		$	331

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 AOCL related to employee benefit plan assets and benefit obligations for the three months ended March 31, 2017 and 2018 were as follows (in thousands):



	Three Months Ended						Three Months Ended
	March 31, 2017						March 31, 2018
Gains (Losses) Included in AOCL	Pension Benefits			Other Postretirement Benefits			Pension Benefits			Other Postretirement Benefits
Beginning balance	$	(58,584	)	$	(7,881	)	$	(97,226	)	$	(6,597	)
Net actuarial loss	—			—			(5,944		)	—
Amortization of prior service credit	(45		)	—			(45		)	—
Amortization of actuarial loss	1,028			200			4,954			160
Settlement cost	1,365			—			—			—
Ending balance	$	(56,236	)	$	(7,681	)	$	(98,261	)	$	(6,437	)

The net periodic benefit costs and AOCL presented in the tables above include one-time corrections made in the first quarter of 2018 resulting from an error in our third-party actuary’s valuation of our pension liabilities and net periodic pension expenses. See Note 1 – Organization, Description of Business and Basis of Presentation for more details regarding this error correction.

Contributions estimated to be paid into the plans in 2018 are $31.7 million and $0.2 million for the pension plans and other postretirement benefit plan, respectively.