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Pension and other postretirement benefits (Tables)
 12 Months Ended
Dec. 31, 2015
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
 
2015
2014
 
2015
2014
Change in benefit obligation:
 
 
 
 
 
Benefit obligation at beginning of year
$
336,292

$
238,319

 
$
158,781

$
134,652

Service cost
6,346

5,605

 
1,970

1,591

Interest cost
13,388

11,629

 
5,985

6,420

Plan amendments
—

(5,308
)
 
(1,758
)
—

Medicare Part D
—

—

 
—

273

Actuarial loss (gain)
(11,429
)
32,772

 
(18,150
)
17,669

Acquisition
—

84,743

 
—

2,857

Benefits paid
(19,247
)
(11,895
)
 
(5,878
)
(4,681
)
Curtailment
1,221

—

 
(8,400
)
—

Settlements
—

(19,573
)
 
—

—

  Benefit obligation at end of year
$
326,571

$
336,292

 
$
132,550

$
158,781
Schedule of Changes in Fair Value of Plan Assets
 
Pension
 
OPEB
 
2015
2014
 
2015
2014
Change in plan assets:
 
 
 
 
 
Fair value of plan assets at beginning of year
$
305,520

$
199,304

 
$
—

$
—

Actual return on plan assets
(11,321
)
16,118

 
—

—

Acquisition
—

115,982

 
—

—

Employer contributions
5,910

5,584

 
5,878

4,681

Benefits paid
(19,247
)
(11,895
)
 
(5,878
)
(4,681
)
Settlements
—

(19,573
)
 
—

—

Fair value of assets at end of year
$
280,862

$
305,520

 
$
—

$
—
Schedule of Amounts Recognized in Balance Sheet
 
Pension
 
OPEB
 
2015
2014
 
2015
2014
Funded status of plans:
 
 
 
 
 
Funded status
$
(45,709
)
$
(30,772
)
 
$
(132,550
)
$
(158,781
)
Amounts recognized in the Consolidated Balance Sheets:
 

 

 
 

 

Non-current assets
$
—

$
30,842

 
$
—

$
—

Current liabilities
(1,743
)
(1,744
)
 
(6,551
)
(6,118
)
Non-current liabilities
(43,966
)
(59,870
)
 
(125,999
)
(152,663
)
Net amount recognized
$
(45,709
)
$
(30,772
)
 
$
(132,550
)
$
(158,781
)
 
 
 
 
 
 
Amounts recognized in accumulated other comprehensive loss (pre-tax):
 

 
 
 

 
Net loss
$
80,514

$
63,360

 
$
49,562

$
79,720

Prior service cost (benefit)
1,211

1,335

 
(9,377
)
(15,751
)
Total
$
81,725

$
64,695

 
$
40,185

$
63,969
Schedule of Net Benefit Cost
Net Periodic Benefit Cost:
 
Year Ended December 31,
 
Pension
 
OPEB
 
2015
2014
2013
 
2015
2014
2013
Service cost
$
6,346

$
5,605

$
4,735

 
$
1,970

$
1,591

$
2,527

Interest cost
13,388

11,629

8,908

 
5,985

6,420

5,681

Expected return on plan assets
(21,241
)
(14,694
)
(10,592
)
 
—

—

—

Amortization of prior service costs
110

77

113

 
(3,728
)
(3,844
)
(3,995
)
Amortization of net loss
3,980

1,907

3,152

 
3,814

3,704

5,022

Settlements
—

4,701

—

 
—

—

—

Curtailment cost (benefit)
1,235

263

(18
)
 
(4,266
)
—

(20
)
Net periodic benefit cost
$
3,818

$
9,488

$
6,298

 
$
3,775

$
7,871

$
9,215
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
 
2015
2014
 
2015
2014
Net loss (gain)
$
21,133

$
24,326

 
$
(26,686
)
$
17,669

Prior service cost (benefit)
—

1,299

 
(1,758
)
—

Amortization of net loss, including recognition due to settlement
(3,980
)
(6,608
)
 
(3,814
)
(3,704
)
Amortization of prior service benefit (cost), including recognition due to curtailment
(124
)
(340
)
 
8,475

3,844

Total amount recognized in other comprehensive loss
17,029

18,677

 
(23,783
)
17,809

Net periodic benefit cost
3,818

9,488

 
3,775

7,871

Total recognized in net periodic benefit cost and other comprehensive loss
$
20,847

$
28,165

 
$
(20,008
)
$
25,680
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 2016:
 
Pension
 
OPEB
Amortization of net loss
$
4,161

 
$
3,518

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 benefit obligations at December 31:
 
Pension
 
OPEB
 
2015
2014
 
2015
2014
Discount rate (1)
4.44%
4.05%
 
4.50%
4.00%
Rate of compensation increase (2)
3%/4%
3%/4%
 
3%/4%
3%/4%
Measurement date
12/31/2015
12/31/2014
 
12/31/2015
12/31/2014


Weighted average assumptions used to determine net periodic benefit cost for the years ended December 31:
 
Pension
 
OPEB
 
2015
2014
2013
 
2015
2014
2013
Measurement date
12/31/2014
12/31/2013
12/31/2012
 
12/31/2014
12/31/2013
12/31/2012
Fiscal year end
12/31/2015
12/31/2014
12/31/2013
 
12/31/2015
12/31/2014
12/31/2013
Discount rate (1)
4.05%
4.89%
4.00%
 
4.00%
4.99%
4.01%
Rate of compensation increase (2)
3%/4%
3%/4%
3%/4%
 
3%/4%
3%/4%
3%/4%
Expected return on plan assets (3)
7.16%
7.25%
7.25%
 
—
—
—

(1)
We use the Ryan Discount Rate Curve ("Ryan Curve") to determine the discount rate.
(2)
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.
For 2013, the rate of compensation increase is 3% per year for the first five years and 4% per year for year six 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 2015:
 
1% Increase
 
1% Decrease
Effect on total of service and interest cost
$
1,403

 
$
(1,108
)
Effect on accumulated postretirement benefit obligation
18,170

 
(15,086
)
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
 
2015 Target
December 31, 2015
December 31, 2014
Equities:
 
 
 
U.S. equities
31%
30%
42%
International equities
21%
20%
18%
Fixed income
48%
50%
40%
 
 
100%
100%
Fair Value of Pension Plans’ assets included under the fair value hierarchy:
As of December 31, 2015
Level 1
Level 2
Level 3
Total
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

As of December 31, 2014
 

 

 

 

Equities:
 

 

 

 

U.S. equities
$
79,148

$
—

$
—

$
79,148

International equities
33,720

—

—

33,720

Fixed income
76,204

—

—

76,204

Plan Receivable (1)
—

—

116,448

116,448

Total
$
189,072

$
—

$
116,448

$
305,520



(1)
Represents the receivable to the Century Aluminum of South Carolina defined benefit plan related to pension funding obligations under the Stock Purchase Agreement which was received in 2015. The plan receivable was considered a Level 3 asset because determining the fair value requires significant unobservable inputs, including census data and various actuarial assumptions.
Schedule Of Expected Benefit Plan Contributions
We expect to make the following contributions for 2016:
 
2016
Expected pension plan contributions
$
1,769

Expected OPEB benefits payments
6,551
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
2016
$
26,438

 
$
6,551

2017
19,755

 
6,789

2018
20,228

 
7,099

2019
21,229

 
7,388

2020
21,655

 
7,508

2021 – 2025
105,301

 
39,031
Schedule of Multiemployer Plans
Century’s participation in the plan for the year ended December 31, 2015, is outlined in the table below.
Fund
Steelworkers Pension Trust
EIN / PN
23-6648508/499
Pension Protection Act Zone Status 2015 (1)
Green
Pension Protection Act Zone Status 2014 (1)
Green
Subject to Financial Improvement/Rehabilitation Plan
No
Contributions of Century Aluminum 2015
$1,618
Contributions of Century Aluminum 2014
$2,164
Contributions of Century Aluminum 2013
$2,171
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 2015 and 2014 is for the plan's year-end December 31, 2014 and December 31, 2013, 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.