Fair value measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Deferred compensation plan assets		$ 260
Total postretirement benefit plan assets	$ 1,696,401	1,329,751
Common collective trust
Assets
Total postretirement benefit plan assets	1,212,114	862,565
Mutual funds
Assets
Total postretirement benefit plan assets	171,198	157,088
Fixed income securities
Assets
Total postretirement benefit plan assets	192,598	175,543
Short-term investments
Assets
Total postretirement benefit plan assets	1,201	1,166
Hedge fund of funds
Assets
Total postretirement benefit plan assets	75,108	71,354
Real estate funds
Assets
Total postretirement benefit plan assets	938	61,249
Cash and accrued income
Assets
Total postretirement benefit plan assets	43,244	786
Derivatives designated as hedging instruments | Commodity contracts
Assets
Derivatives	(1,625)	(1,571)
Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	1,058	(1,712)
Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	54	166
Assets measured at NAV
Assets
Deferred compensation plan assets		0
Total postretirement benefit plan assets	1,288,160	995,168
Assets measured at NAV | Common collective trust
Assets
Total postretirement benefit plan assets	1,212,114	862,565
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	0	0
Assets measured at NAV | Short-term investments
Assets
Total postretirement benefit plan assets
Assets measured at NAV | Hedge fund of funds
Assets
Total postretirement benefit plan assets	75,108	71,354
Assets measured at NAV | Real estate funds
Assets
Total postretirement benefit plan assets	938	61,249
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	0	0
Assets measured at NAV | Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	0	0
Assets measured at NAV | Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	0	0
Level 1
Assets
Deferred compensation plan assets		260
Total postretirement benefit plan assets	43,267	824
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	0
Level 1 | Short-term investments
Assets
Total postretirement benefit plan assets	23	38
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	43,244	786
Level 1 | Derivatives designated as hedging instruments | Commodity contracts
Assets
Derivatives	0	0
Level 1 | Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	0	0
Level 1 | Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	0	0
Level 2
Assets
Deferred compensation plan assets		0
Total postretirement benefit plan assets	364,974	333,759
Level 2 | Common collective trust
Assets
Total postretirement benefit plan assets	0	0
Level 2 | Mutual funds
Assets
Total postretirement benefit plan assets	171,198	157,088
Level 2 | Fixed income securities
Assets
Total postretirement benefit plan assets	192,598	175,543
Level 2 | Short-term investments
Assets
Total postretirement benefit plan assets	1,178	1,128
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	(1,625)	(1,571)
Level 2 | Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	1,058	(1,712)
Level 2 | Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	54	166
Level 3
Assets
Deferred compensation plan assets		0
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	0	0
Level 3 | Derivatives designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	0	0
Level 3 | Derivatives not designated as hedging instruments | Foreign exchange contracts
Assets
Derivatives	$ 0	$ 0