Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 46	$ 31	$ 86	$ 64
Interest cost	4,206	4,405	8,570	8,845
Expected return on assets	(5,753)	(5,484)	(11,126)	(10,609)
Amortization of prior service cost (credit)	18	18	36	36
Amortization of net actuarial loss	1,813	1,121	3,548	2,434
Curtailment loss	258	0	258	0
Net periodic benefit cost (credit), including amount capitalized	588	91	1,372	770
Less amount capitalized	53	73	129	168
Net periodic benefit cost	535	18	1,243	602
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	425	380	908	759
Interest cost	889	924	1,803	1,782
Expected return on assets	(1,223)	(1,242)	(2,398)	(2,309)
Amortization of prior service cost (credit)	(343)	(348)	(685)	(696)
Amortization of net actuarial loss	553	6	1,014	324
Curtailment loss	0	0	0	0
Net periodic benefit cost (credit), including amount capitalized	301	(280)	642	(140)
Less amount capitalized	33	(19)	62	10
Net periodic benefit cost	268	(261)	580	(150)
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,900	$ 1,600	$ 3,600	$ 3,300