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Fair value measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Assets |  |
Total postretirement benefit plan assets | $ 431,047 | $ 1,813,135
Common collective trust |  |
Assets |  |
Total postretirement benefit plan assets | 8,882 | 7,750
Mutual funds |  |
Assets |  |
Total postretirement benefit plan assets | 118,559 | 152,756
Fixed income securities |  |
Assets |  |
Total postretirement benefit plan assets | 292,883 | 1,533,149
Short-term investments |  |
Assets |  |
Total postretirement benefit plan assets | 1,211 | 1,223
Hedge fund of funds |  |
Assets |  |
Total postretirement benefit plan assets |  | 67
Real estate funds |  |
Assets |  |
Total postretirement benefit plan assets | 592 | 552
Cash and accrued income |  |
Assets |  |
Total postretirement benefit plan assets | 8,920 | 117,638
Derivatives designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 1,491 | (647)
Derivatives designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | (121) | 602
Derivatives not designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 683 | 484
Derivatives not designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | (41) | 115
Derivatives not designated as hedging instruments | Interest Rate Lock Contract |  |
Assets |  |
Derivatives, net | (550) |
Assets measured at NAV |  |
Assets |  |
Total postretirement benefit plan assets | 50,594 | 1,306,195
Assets measured at NAV | Common collective trust |  |
Assets |  |
Total postretirement benefit plan assets | 8,882 | 7,750
Assets measured at NAV | Mutual funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Assets measured at NAV | Fixed income securities |  |
Assets |  |
Total postretirement benefit plan assets | 41,120 | 1,297,826
Assets measured at NAV | Short-term investments |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Assets measured at NAV | Hedge fund of funds |  |
Assets |  |
Total postretirement benefit plan assets |  | 67
Assets measured at NAV | Real estate funds |  |
Assets |  |
Total postretirement benefit plan assets | 592 | 552
Assets measured at NAV | Cash and accrued income |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Assets measured at NAV | Derivatives designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Assets measured at NAV | Derivatives designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Assets measured at NAV | Derivatives not designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net |  |
Assets measured at NAV | Derivatives not designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Assets measured at NAV | Derivatives not designated as hedging instruments | Interest Rate Lock Contract |  |
Assets |  |
Derivatives, net |  |
Level 1 |  |
Assets |  |
Total postretirement benefit plan assets | 8,920 | 117,655
Level 1 | Common collective trust |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 1 | Mutual funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 1 | Fixed income securities |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 17
Level 1 | Short-term investments |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 1 | Hedge fund of funds |  |
Assets |  |
Total postretirement benefit plan assets |  | 0
Level 1 | Real estate funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 1 | Cash and accrued income |  |
Assets |  |
Total postretirement benefit plan assets | 8,920 | 117,638
Level 1 | Derivatives designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Level 1 | Derivatives designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Level 1 | Derivatives not designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net |  |
Level 1 | Derivatives not designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Level 1 | Derivatives not designated as hedging instruments | Interest Rate Lock Contract |  |
Assets |  |
Derivatives, net | 0 |
Level 2 |  |
Assets |  |
Total postretirement benefit plan assets | 371,533 | 389,285
Level 2 | Common collective trust |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 2 | Mutual funds |  |
Assets |  |
Total postretirement benefit plan assets | 118,559 | 152,756
Level 2 | Fixed income securities |  |
Assets |  |
Total postretirement benefit plan assets | 251,763 | 235,306
Level 2 | Short-term investments |  |
Assets |  |
Total postretirement benefit plan assets | 1,211 | 1,223
Level 2 | Hedge fund of funds |  |
Assets |  |
Total postretirement benefit plan assets |  | 0
Level 2 | Real estate funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 2 | Cash and accrued income |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 2 | Derivatives designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 1,491 | (647)
Level 2 | Derivatives designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | (121) | 602
Level 2 | Derivatives not designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 683 | 484
Level 2 | Derivatives not designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | (41) | 115
Level 2 | Derivatives not designated as hedging instruments | Interest Rate Lock Contract |  |
Assets |  |
Derivatives, net | (550) |
Level 3 |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Common collective trust |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Mutual funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Fixed income securities |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Short-term investments |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Hedge fund of funds |  |
Assets |  |
Total postretirement benefit plan assets |  | 0
Level 3 | Real estate funds |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Cash and accrued income |  |
Assets |  |
Total postretirement benefit plan assets | 0 | 0
Level 3 | Derivatives designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Level 3 | Derivatives designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | 0
Level 3 | Derivatives not designated as hedging instruments | Commodity Contracts |  |
Assets |  |
Derivatives, net |  |
Level 3 | Derivatives not designated as hedging instruments | Foreign Exchange Contracts |  |
Assets |  |
Derivatives, net | 0 | $ 0
Level 3 | Derivatives not designated as hedging instruments | Interest Rate Lock Contract |  |
Assets |  |
Derivatives, net | $ 0 |

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