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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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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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