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Employee benefit plans | 9 Months Ended
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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.

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