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Pension and other postretirement benefits (Tables)
 12 Months Ended
Dec. 31, 2017
Retirement Benefits [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
 
2017
 
2016
 
2017
 
2016
Change in benefit obligation:
 
 
 
 
 
 
 
Benefit obligation at beginning of year
$
328,047

 
$
326,571

 
$
133,856

 
$
132,550

Service cost
4,384

 
4,651

 
801

 
1,003

Interest cost
13,310

 
13,892

 
5,317

 
5,595

Plan amendments
—

 
—

 
(27,395
)
 
—

Actuarial loss (gain)
18,523

 
12,761

 
14,698

 
1,919

Medicare Part D
—

 
—

 
406

 
38

Benefits paid
(20,156
)
 
(29,828
)
 
(6,919
)
 
(7,249
)
  Benefit obligation at end of year
$
344,108

 
$
328,047

 
$
120,764

 
$
133,856
Schedule of Changes in Fair Value of Plan Assets
 
Pension
 
OPEB
 
2017
 
2016
 
2017
 
2016
Change in plan assets:
 
 
 
 
 
 
 
Fair value of plan assets at beginning of year
$
276,741

 
$
280,862

 
$
—

 
$
—

Actual return on plan assets
45,027

 
23,932

 
—

 
—

Employer contributions
1,755

 
1,775

 
6,513

 
7,211

Medicare Part D subsidy received
—

 
—

 
406

 
38

Benefits paid
(20,156
)
 
(29,828
)
 
(6,919
)
 
(7,249
)
Fair value of assets at end of year
$
303,367

 
$
276,741

 
$
—

 
$
—
Schedule of Amounts Recognized in Balance Sheet
 
Pension
 
OPEB
 
2017
 
2016
 
2017
 
2016
Funded status of plans:
 
 
 
 
 
 
 
Funded status
$
(40,741
)
 
$
(51,306
)
 
$
(120,764
)
 
$
(133,856
)
Amounts recognized in the Consolidated Balance Sheets:
 

 
 

 
 

 
 

Current liabilities
(1,812
)
 
(1,813
)
 
(7,459
)
 
(7,501
)
Non-current liabilities
(38,929
)
 
(49,493
)
 
(113,305
)
 
(126,355
)
Net amount recognized
$
(40,741
)
 
$
(51,306
)
 
$
(120,764
)
 
$
(133,856
)
Amounts recognized in accumulated other comprehensive loss (pre-tax):
 

 
 
 
 

 
 
Net loss
$
71,199

 
$
83,451

 
$
58,434

 
$
47,957

Prior service cost (benefit)
998

 
1,104

 
(28,935
)
 
(6,595
)
Total
$
72,197

 
$
84,555

 
$
29,499

 
$
41,362
Schedule of Net Benefit Cost
Net Periodic Benefit Cost:
 
Year Ended December 31,
 
Pension
 
OPEB
 
2017
 
2016
 
2015
 
2017
 
2016
 
2015
Service cost
$
4,384

 
$
4,651

 
$
6,346

 
$
801

 
$
1,003

 
$
1,970

Interest cost
13,310

 
13,892

 
13,388

 
5,317

 
5,595

 
5,985

Expected return on plan assets
(18,997
)
 
(18,774
)
 
(21,241
)
 
—

 
—

 
—

Amortization of prior service costs
106

 
106

 
110

 
(3,692
)
 
(2,781
)
 
(3,728
)
Amortization of net loss
4,745

 
4,666

 
3,980

 
3,865

 
3,537

 
3,814

Curtailment cost (benefit)
—

 
—

 
1,235

 
(1,364
)
 
—

 
(4,266
)
Net periodic benefit cost
$
3,548

 
$
4,541

 
$
3,818

 
$
4,927

 
$
7,354

 
$
3,775
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
 
2017
 
2016
 
2017
 
2016
Net loss (gain)
$
(7,507
)
 
$
7,603

 
$
14,699

 
$
1,919

Prior service cost (benefit)
—

 
—

 
(27,396
)
 
—

Amortization of net loss, including recognition due to settlement
(4,745
)
 
(4,666
)
 
(3,865
)
 
(3,537
)
Amortization of prior service benefit (cost), including recognition due to curtailment
(106
)
 
(106
)
 
5,056

 
2,781

Total amount recognized in other comprehensive loss
(12,358
)
 
2,831

 
(11,506
)
 
1,163

Net periodic benefit cost
3,548

 
4,541

 
4,927

 
7,354

Total recognized in net periodic benefit cost and other comprehensive loss
$
(8,810
)
 
$
7,372

 
$
(6,579
)
 
$
8,517
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 2018
 
Pension
 
OPEB
Amortization of net loss
$
5,966

 
$
3,722

Amortization of prior service cost (benefit)
106

 
(7,258
)
Schedule of Weighted Average Assumptions Used in Calculating Benefit Obligation and Net Periodic Benefit Cost
 
Weighted average assumptions used to determine benefit obligations at December 31:
 
Pension
 
OPEB
 
2017
 
2016
 
2017
 
2016
Discount rate (1)
3.69%
 
4.19%
 
3.66%
 
4.20%
Rate of compensation increase (2)
3%/4%
 
3%/4%
 
3%/4%
 
3%/4%
Measurement date
12/31/2017
 
12/31/2016
 
12/31/2017
 
12/31/2016
Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
 
Pension
 
OPEB
 
2017
 
2016
 
2015
 
2017
 
2016
 
2015
Measurement date
12/31/2016
 
12/31/2015
 
12/31/2014
 
12/31/2016
 
12/31/2015
 
12/31/2014
Fiscal year end
12/31/2017
 
12/31/2016
 
12/31/2015
 
12/31/2017
 
12/31/2016
 
12/31/2015
Discount rate (1)
4.15%
 
4.44%
 
4.05%
 
4.05%
 
4.50%
 
4.00%
Rate of compensation increase (2)
3%/4%
 
3%/4%
 
3%/4%
 
3%/4%
 
3%/4%
 
3%/4%
Expected return on plan assets (3)
6.82%
 
7.10%
 
7.16%
 
—
 
—
 
—

(1) 
We use the Ryan Curve to determine the discount rate.
(2) 
For 2017, the rate of compensation increase is 3% per year for the first two years and 4% per year for year three and thereafter. For 2016, the rate of compensation increase is 3% per year for the first three years and 4% per year for year four and thereafter. For 2015, 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.
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 2017:
 
1% Increase
 
1% Decrease
Effect on total of service and interest cost
$
784

 
$
(653
)
Effect on accumulated postretirement benefit obligation
12,461

 
(10,611
)
Schedule of Allocation of Plan Assets
The Pension Plans’ weighted average long-term strategic asset allocation policy targets are as follows:
 
Pension Plan Asset Allocation
 
2017 Target
 
December 31, 2017
 
December 31, 2016
Equities:
 
 
 
 
 
U.S. equities
29%
 
29%
 
34%
International equities
26%
 
26%
 
22%
Fixed income
45%
 
45%
 
44%
 
 
 
100%
 
100%
Fair Value of Pension Plans’ assets included under the fair value hierarchy:
As of December 31, 2017
Level 1
 
Level 2
 
Level 3
 
Total
Equities:
 
 
 
 
 
 
 
U.S. equities
$
86,513

 
$
—

 
$
—

 
$
86,513

International equities
79,120

 
—

 
—

 
79,120

Fixed income
137,734

 
—

 
—

 
137,734

Total
$
303,367

 
$
—

 
$
—

 
$
303,367

As of December 31, 2016
 

 
 

 
 

 
 

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
Schedule Of Expected Benefit Plan Contributions
We expect to make the following contributions for 2018:
 
2018
Expected pension plan contributions
$
1,812

Expected OPEB benefits payments
7,459
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
2018
$
20,261

 
$
7,459

2019
20,802

 
7,559

2020
21,057

 
7,625

2021
21,030

 
7,847

2022
21,382

 
7,876

2023 – 2027
101,437

 
38,085
Schedule of Multiemployer Plans
Century’s participation in the plan for the year ended December 31, 2017, is outlined in the table below.
Fund
 
Steelworkers Pension Trust
EIN / PN
 
23-6648508/499
Pension Protection Act Zone Status 2017 (1)
 
Green
Pension Protection Act Zone Status 2016 (1)
 
Green
Subject to Financial Improvement/Rehabilitation Plan
 
No
Contributions of Century Aluminum 2017
 
$767
Contributions of Century Aluminum 2016
 
$788
Contributions of Century Aluminum 2015
 
$1,618
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 2017 and 2016 is for the plan's year-end December 31, 2016 and December 31, 2015, 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.