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Employee benefits - Components of net periodic benefit cost (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plans      
Personnel costs:      
Service cost $ 0 $ 0 $ 0
Other operating expenses      
Interest cost 28,439,000 25,493,000 25,889,000
Expected return on plan assets (32,388,000) (40,240,000) (42,752,000)
Amortization of prior service cost (credit) 0 0 0
Amortization of net loss 23,508,000 20,260,000 21,859,000
Net periodic benefit (credit) cost 19,559,000 5,513,000 4,996,000
Termination benefit cost 0 0 0
Total net periodic pension cost 19,559,000 5,513,000 4,996,000
OPEB Plan      
Personnel costs:      
Service cost 759,000 1,028,000 1,026,000
Other operating expenses      
Interest cost 5,955,000 5,562,000 5,703,000
Expected return on plan assets 0 0 0
Amortization of prior service cost (credit) 0 (3,470,000) (3,800,000)
Amortization of net loss 0 1,282,000 569,000
Net periodic benefit (credit) cost 6,714,000 4,402,000 3,498,000
Termination benefit cost 0 1,790,000 0
Total net periodic pension cost $ 6,714,000 $ 6,192,000 $ 3,498,000