Employee benefit plans

9 Months Ended

Sep. 30, 2015

Compensation and Retirement Disclosure [Abstract]

Pension and other postretirement benefit plans

Employee benefit plans

Pension and other postretirement plans

The Company has noncontributory defined benefit pension plans and other postretirement benefit plans for certain eligible employees. Components of net periodic benefit cost (credit) for the Company's pension and other postretirement benefit plans were as follows:

	Pension Benefits						Other Postretirement Benefits
Three Months Ended September 30,	2015			2014			2015			2014
	(In thousands)
Components of net periodic benefit cost (credit):
Service cost	$	—		$	32		$	454		$	379
Interest cost	4,285			4,420			902			919
Expected return on assets	(5,563		)	(5,304		)	(1,199		)	(1,154		)
Amortization of prior service cost (credit)	—			18			(343		)	(348		)
Amortization of net actuarial loss	1,734			1,217			511			162
Net periodic benefit cost (credit), including amount capitalized	456			383			325			(42		)
Less amount capitalized	90			27			36			(65		)
Net periodic benefit cost (credit)	$	366		$	356		$	289		$	23

	Pension Benefits						Other Postretirement Benefits
Nine Months Ended September 30,	2015			2014			2015			2014
	(In thousands)
Components of net periodic benefit cost (credit):
Service cost	$	86		$	96		$	1,362		$	1,138
Interest cost	12,855			13,265			2,705			2,701
Expected return on assets	(16,689		)	(15,913		)	(3,597		)	(3,463		)
Amortization of prior service cost (credit)	36			54			(1,028		)	(1,044		)
Amortization of net actuarial loss	5,282			3,651			1,525			486
Curtailment loss	258			—			—			—
Net periodic benefit cost (credit), including amount capitalized	1,828			1,153			967			(182		)
Less amount capitalized	219			195			98			(55		)
Net periodic benefit cost (credit)	$	1,609		$	958		$	869		$	(127	)

Nonqualified benefit plans

In addition to the qualified plan defined pension benefits reflected in the table, the Company has unfunded, nonqualified benefit plans for executive officers and certain key management employees that generally provide for defined benefit payments at age 65 following the employee's retirement or to their beneficiaries upon death for a 15-year period. The Company's net periodic benefit cost for these plans for the three and nine months ended September 30, 2015, was $1.7 million and $5.3 million, respectively. The Company's net periodic benefit cost for these plans for the three and nine months ended September 30, 2014, was $1.7 million and $5.0 million, respectively.

Multiemployer plans

On September 24, 2014, Knife River provided notice to the Operating Engineers Local 800 & WY Contractors Association, Inc. Pension Plan for Wyoming that it was withdrawing from the plan effective October 26, 2014. The plan administrator will determine Knife River's withdrawal liability. For the three months ended March 31, 2015, the Company accrued an additional withdrawal liability of approximately $2.4 million. The cumulative withdrawal liability is currently estimated at $16.4 million which has been accrued on the Consolidated Balance Sheets. The assessed withdrawal liability for this plan may be significantly different from the current estimate.