Fair value measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Total postretirement benefit plan assets	$ 1,813,135	$ 1,696,401
Common collective trust
Assets
Total postretirement benefit plan assets	7,750	1,212,114
Mutual funds
Assets
Total postretirement benefit plan assets	152,756	171,198
Fixed income securities
Assets
Total postretirement benefit plan assets	1,533,149	192,598
Short-term investments
Assets
Total postretirement benefit plan assets	1,223	1,201
Hedge fund of funds
Assets
Total postretirement benefit plan assets	67	75,108
Real estate funds
Assets
Total postretirement benefit plan assets	552	938
Cash and accrued income
Assets
Total postretirement benefit plan assets	117,638	43,244
Derivatives designated as hedging instruments | Commodity contracts
Assets
Derivatives, net	(647)	(1,625)
Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives, net	602	1,058
Derivatives not designated as hedging instruments | Commodity contracts
Assets
Derivatives, net	484
Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives, net	115	54
Assets measured at NAV
Assets
Total postretirement benefit plan assets	1,306,195	1,288,160
Assets measured at NAV | Common collective trust
Assets
Total postretirement benefit plan assets	7,750	1,212,114
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	1,297,826	0
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	75,108
Assets measured at NAV | Real estate funds
Assets
Total postretirement benefit plan assets	552	938
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
Level 1
Assets
Total postretirement benefit plan assets	117,655	43,267
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	17	0
Level 1 | Short-term investments
Assets
Total postretirement benefit plan assets	0	23
Level 1 | Hedge fund of funds
Assets
Total postretirement benefit plan assets	0	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	117,638	43,244
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 2
Assets
Total postretirement benefit plan assets	389,285	364,974
Level 2 | Common collective trust
Assets
Total postretirement benefit plan assets	0	0
Level 2 | Mutual funds
Assets
Total postretirement benefit plan assets	152,756	171,198
Level 2 | Fixed income securities
Assets
Total postretirement benefit plan assets	235,306	192,598
Level 2 | Short-term investments
Assets
Total postretirement benefit plan assets	1,223	1,178
Level 2 | Hedge fund of funds
Assets
Total postretirement benefit plan assets	0	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	(647)	(1,625)
Level 2 | Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives, net	602	1,058
Level 2 | Derivatives not designated as hedging instruments | Commodity contracts
Assets
Derivatives, net	484
Level 2 | Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives, net	115	54
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	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