Employee benefit plans

6 Months Ended

Jun. 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 for the Company's pension and other postretirement benefit plans were as follows:

							Other
							Postretirement
	Pension Benefits						Benefits
Three Months Ended June 30,	2015			2014			2015			2014
	(In thousands)
Components of net periodic benefit cost (credit):
Service cost	$	46		$	31		$	425		$	380
Interest cost	4,206			4,405			889			924
Expected return on assets	(5,753		)	(5,484		)	(1,223		)	(1,242		)
Amortization of prior service cost (credit)	18			18			(343		)	(348		)
Amortization of net actuarial loss	1,813			1,121			553			6
Curtailment loss	258			—			—			—
Net periodic benefit cost (credit), including amount capitalized	588			91			301			(280		)
Less amount capitalized	53			73			33			(19		)
Net periodic benefit cost (credit)	$	535		$	18		$	268		$	(261	)

							Other
							Postretirement
	Pension Benefits						Benefits
Six Months Ended June 30,	2015			2014			2015			2014
	(In thousands)
Components of net periodic benefit cost (credit):
Service cost	$	86		$	64		$	908		$	759
Interest cost	8,570			8,845			1,803			1,782
Expected return on assets	(11,126		)	(10,609		)	(2,398		)	(2,309		)
Amortization of prior service cost (credit)	36			36			(685		)	(696		)
Amortization of net actuarial loss	3,548			2,434			1,014			324
Curtailment loss	258			—			—			—
Net periodic benefit cost (credit), including amount capitalized	1,372			770			642			(140		)
Less amount capitalized	129			168			62			10
Net periodic benefit cost (credit)	$	1,243		$	602		$	580		$	(150	)

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 six months ended June 30, 2015, was $1.9 million and $3.6 million, respectively. The Company's net periodic benefit cost for these plans for the three and six months ended June 30, 2014, was $1.6 million and $3.3 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 (approximately $1.5 million after tax). The total withdrawal liability is currently estimated at $16.4 million (approximately $9.8 million after tax). The assessed withdrawal liability for this plan may be significantly different from the current estimate.