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Pension and other postretirement benefits (Tables) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract] |  |
Schedule of Changes in Projected Benefit Obligations |  | The change in benefit obligations and change in plan assets as of December 31 are as follows:
| Pension |  | OPEB
| 2016 | 2015 |  | 2016 | 2015
Change in benefit obligation: |  |  |  |  |
Benefit obligation at beginning of year | $ | 326,571 |  | $ | 336,292 |  |  | $ | 132,550 |  | $ | 158,781 |
Service cost | 4,651 |  | 6,346 |  |  | 1,003 |  | 1,970 |
Interest cost | 13,892 |  | 13,388 |  |  | 5,595 |  | 5,985 |
Plan amendments | — |  | — |  |  | — |  | (1,758 | )
Actuarial loss (gain) | 12,761 |  | (11,429 | ) |  | 1,919 |  | (18,150 | )
Medicare Part D | — |  | — |  |  | 38 |  | — |
Benefits paid | (29,828 | ) | (19,247 | ) |  | (7,249 | ) | (5,878 | )
Curtailment | — |  | 1,221 |  |  | — |  | (8,400 | )
Benefit obligation at end of year | $ | 328,047 |  | $ | 326,571 |  |  | $ | 133,856 |  | $ | 132,550 |
Schedule of Changes in Fair Value of Plan Assets |  |
| Pension |  | OPEB
| 2016 | 2015 |  | 2016 | 2015
Change in plan assets: |  |  |  |  |
Fair value of plan assets at beginning of year | $ | 280,862 |  | $ | 305,520 |  |  | $ | — |  | $ | — |
Actual return on plan assets | 23,932 |  | (11,321 | ) |  | — |  | — |
Employer contributions | 1,775 |  | 5,910 |  |  | 7,211 |  | 5,878 |
Medicare Part D subsidy received | — |  | — |  |  | 38 |  | — |
Benefits paid | (29,828 | ) | (19,247 | ) |  | (7,249 | ) | (5,878 | )
Fair value of assets at end of year | $ | 276,741 |  | $ | 280,862 |  |  | $ | — |  | $ | — |
Schedule of Amounts Recognized in Balance Sheet |  |
| Pension |  | OPEB
| 2016 | 2015 |  | 2016 | 2015
Funded status of plans: |  |  |  |  |
Funded status | $ | (51,306 | ) | $ | (45,709 | ) |  | $ | (133,856 | ) | $ | (132,550 | )
Amounts recognized in the Consolidated Balance Sheets: |  |  |  |  |  |  |  |  |
Current liabilities | (1,813 | ) | (1,743 | ) |  | (7,501 | ) | (6,551 | )
Non-current liabilities | (49,493 | ) | (43,966 | ) |  | (126,355 | ) | (125,999 | )
Net amount recognized | $ | (51,306 | ) | $ | (45,709 | ) |  | $ | (133,856 | ) | $ | (132,550 | )
Amounts recognized in accumulated other comprehensive loss (pre-tax): |  |  |  |  |  |  |
Net loss | $ | 83,451 |  | $ | 80,514 |  |  | $ | 47,957 |  | $ | 49,562 |
Prior service cost (benefit) | 1,104 |  | 1,211 |  |  | (6,595 | ) | (9,377 | )
Total | $ | 84,555 |  | $ | 81,725 |  |  | $ | 41,362 |  | $ | 40,185 |
Schedule of Net Benefit Cost |
| Pension Benefits
|  | Three months ended March 31,
|  | 2017 | 2016
Service cost |  | $ | 1,224 |  | $ | 1,270 |
Interest cost |  | 3,332 |  | 3,480 |
Expected return on plan assets |  | (4,749 | ) | (4,816 | )
Amortization of prior service costs |  | 27 |  | 28 |
Amortization of net loss |  | 1,232 |  | 1,043 |
Net periodic benefit cost |  | $ | 1,066 |  | $ | 1,005 |
| Other Postretirement Benefits ("OPEB")
|  | Three months ended March 31,
|  | 2017 | 2016
Service cost |  | $ | 247 |  | $ | 330 |
Interest cost |  | 1,366 |  | 1,456 |
Amortization of prior service cost |  | (338 | ) | (695 | )
Amortization of net loss |  | 911 |  | 876 |
Net periodic benefit cost |  | $ | 2,186 |  | $ | 1,967 |
| Net Periodic Benefit Cost:
| Year Ended December 31,
| Pension |  | OPEB
| 2016 | 2015 | 2014 |  | 2016 | 2015 | 2014
Service cost | $ | 4,651 |  | $ | 6,346 |  | $ | 5,605 |  |  | $ | 1,003 |  | $ | 1,970 |  | $ | 1,591 |
Interest cost | 13,892 |  | 13,388 |  | 11,629 |  |  | 5,595 |  | 5,985 |  | 6,420 |
Expected return on plan assets | (18,774 | ) | (21,241 | ) | (14,694 | ) |  | — |  | — |  | — |
Amortization of prior service costs | 106 |  | 110 |  | 77 |  |  | (2,781 | ) | (3,728 | ) | (3,844 | )
Amortization of net loss | 4,666 |  | 3,980 |  | 1,907 |  |  | 3,537 |  | 3,814 |  | 3,704 |
Settlements | — |  | — |  | 4,701 |  |  | — |  | — |  | — |
Curtailment cost (benefit) | — |  | 1,235 |  | 263 |  |  | — |  | (4,266 | ) | — |
Net periodic benefit cost | $ | 4,541 |  | $ | 3,818 |  | $ | 9,488 |  |  | $ | 7,354 |  | $ | 3,775 |  | $ | 7,871 |
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) |  | Other changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss (pre-tax):
| Year Ended December 31,
| Pension |  | OPEB
| 2016 | 2015 |  | 2016 | 2015
Net loss (gain) | $ | 7,603 |  | $ | 21,133 |  |  | $ | 1,919 |  | $ | (26,686 | )
Prior service cost (benefit) | — |  | — |  |  | — |  | (1,758 | )
Amortization of net loss, including recognition due to settlement | (4,666 | ) | (3,980 | ) |  | (3,537 | ) | (3,814 | )
Amortization of prior service benefit (cost), including recognition due to curtailment | (106 | ) | (124 | ) |  | 2,781 |  | 8,475 |
Total amount recognized in other comprehensive loss | 2,831 |  | 17,029 |  |  | 1,163 |  | (23,783 | )
Net periodic benefit cost | 4,541 |  | 3,818 |  |  | 7,354 |  | 3,775 |
Total recognized in net periodic benefit cost and other comprehensive loss | $ | 7,372 |  | $ | 20,847 |  |  | $ | 8,517 |  | $ | (20,008 | )
Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year |  |
Amounts in accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost during 2017
| Pension |  | OPEB
Amortization of net loss | $ | 4,926 |  |  | $ | 3,645 |
Amortization of prior service cost (benefit) | 106 |  |  | (2,781 | )
Weighted Average Assumptions Used in Calculating Benefit Obligation and Net Periodic Benefit Cost |  | Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
| Pension |  | OPEB
| 2016 | 2015 | 2014 |  | 2016 | 2015 | 2014
Measurement date | 12/31/2015 | 12/31/2014 | 12/31/2013 |  | 12/31/2015 | 12/31/2014 | 12/31/2013
Fiscal year end | 12/31/2016 | 12/31/2015 | 12/31/2014 |  | 12/31/2016 | 12/31/2015 | 12/31/2014
Discount rate (1) | 4.44% | 4.05% | 4.89% |  | 4.50% | 4.00% | 4.99%
Rate of compensation increase (2) | 3%/4% | 3%/4% | 3%/4% |  | 3%/4% | 3%/4% | 3%/4%
Expected return on plan assets (3) | 7.10% | 7.16% | 7.25% |  | — | — | —
(1) | We use the Ryan Above Median Discount Rate Curve ("Ryan Curve") to determine the discount rate.
(2) | For 2016, the rate of compensation increase is 3% per year for the first two years and 4% per year for year three and thereafter. For 2015, the rate of compensation increase is 3% per year for the first three years and 4% per year for year four and thereafter. For 2014, the rate of compensation increase is 3% per year for the first four years and 4% per year for year five and thereafter.
(3) | The rate for each of our defined benefit plans was selected by taking into account our expected asset mix and is based on historical performance as well as expected future rates of return on plan assets.
Weighted average assumptions used to determine benefit obligations at December 31:
| Pension |  | OPEB
| 2016 | 2015 |  | 2016 | 2015
Discount rate (1) | 4.19% | 4.44% |  | 4.20% | 4.50%
Rate of compensation increase (2) | 3%/4% | 3%/4% |  | 3%/4% | 3%/4%
Measurement date | 12/31/2016 | 12/31/2015 |  | 12/31/2016 | 12/31/2015
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates |  | A one-percentage-point change in the assumed health care cost trend rate would have had the following effects in 2016:
| 1% Increase |  | 1% Decrease
Effect on total of service and interest cost | $ | 933 |  |  | $ | (769 | )
Effect on accumulated postretirement benefit obligation | 16,968 |  |  | (14,290 | )
Schedule of Allocation of Plan Assets |  | Fair Value of Pension Plans’ assets included under the fair value hierarchy:
As of December 31, 2016 | Level 1 | Level 2 | Level 3 | Total
Equities: |  |  |  |
U.S. equities | $ | 93,773 |  | $ | — |  | $ | — |  | $ | 93,773 |
International equities | 61,453 |  | — |  | — |  | 61,453 |
Fixed income | 121,515 |  | — |  | — |  | 121,515 |
Total | $ | 276,741 |  | $ | — |  | $ | — |  | $ | 276,741 |
As of December 31, 2015 |  |  |  |  |  |  |  |
Equities: |  |  |  |  |  |  |  |
U.S. equities | $ | 86,723 |  | $ | — |  | $ | — |  | $ | 86,723 |
International equities | 54,769 |  | — |  | — |  | 54,769 |
Fixed income | 139,370 |  | — |  | — |  | 139,370 |
Total | $ | 280,862 |  | $ | — |  | $ | — |  | $ | 280,862 |
The Pension Plans’ weighted average long-term strategic asset allocation policy targets are as follows:
| Pension Plan Asset Allocation
| 2016 Target | December 31, 2016 | December 31, 2015
Equities: |  |  |
U.S. equities | 33% | 34% | 30%
International equities | 22% | 22% | 20%
Fixed income | 45% | 44% | 50%
|  | 100% | 100%
Schedule Of Expected Benefit Plan Contributions |  | We expect to make the following contributions for 2017:
| 2017
Expected pension plan contributions | $ | 1,813 |
Expected OPEB benefits payments | 7,501 |
Schedule of Expected Benefit Payments |  | The following table provides the estimated future benefit payments for the pension and other postretirement benefit plans:
| Pension Benefits |  | OPEB Benefits
2017 | $ | 19,705 |  |  | $ | 7,501 |
2018 | 19,562 |  |  | 7,519 |
2019 | 20,429 |  |  | 7,855 |
2020 | 20,929 |  |  | 8,055 |
2021 | 20,819 |  |  | 8,287 |
2022 – 2026 | 102,567 |  |  | 35,560 |
Schedule of Multiemployer Plans |  | Century’s participation in the plan for the year ended December 31, 2016, is outlined in the table below.
Fund | Steelworkers Pension Trust
EIN / PN | 23-6648508/499
Pension Protection Act Zone Status 2016 (1) | Green
Pension Protection Act Zone Status 2015 (1) | Green
Subject to Financial Improvement/Rehabilitation Plan | No
Contributions of Century Aluminum 2016 | $788
Contributions of Century Aluminum 2015 | $1,618
Contributions of Century Aluminum 2014 | $2,164
Withdrawal from Plan Probable | No
Surcharge Imposed | No
Expiration Date of Collective Bargaining Agreement | April 1, 2020
(1) | The most recent Pension Protection Act zone status available in 2016 and 2015 is for the plan's year-end December 31, 2015 and December 31, 2014, respectively. The zone status is based on information that Century received from the plan as well as publicly available information per the Department of Labor and is certified by the plan’s actuary. Among other factors, plans in the green zone are at least 80 percent funded.

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