Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	4,014	4,390
Expected return on assets	(5,029)	(5,280)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	1,793	1,593
Net periodic benefit cost, including amount capitalized	778	703
Less amount capitalized	107	81
Net periodic benefit cost	671	622
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	447	450
Interest cost	808	949
Expected return on assets	(1,145)	(1,149)
Amortization of prior service credit	(343)	(343)
Amortization of net actuarial loss	336	448
Net periodic benefit cost, including amount capitalized	103	355
Less amount capitalized	(39)	34
Net periodic benefit cost	142	321
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Curtailment gain		3,300
Net periodic benefit cost	$ 1,200	$ (1,900)