Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 31	$ 37	$ 64	$ 77
Interest cost	4,405	4,106	8,845	8,124
Expected return on assets	(5,484)	(4,875)	(10,609)	(9,958)
Amortization of prior service cost (credit)	18	18	36	36
Amortization of net actuarial loss	1,121	1,716	2,434	3,580
Net periodic benefit cost, including amount capitalized	91	1,002	770	1,859
Less amount capitalized	73	158	168	268
Net periodic benefit cost (credit)	18	844	602	1,591
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	380	334	759	838
Interest cost	924	667	1,782	1,607
Expected return on assets	(1,242)	(1,065)	(2,309)	(2,172)
Amortization of prior service cost (credit)	(348)	(364)	(696)	(728)
Amortization of net actuarial loss	6	407	324	1,078
Net periodic benefit cost, including amount capitalized	(280)	(21)	(140)	623
Less amount capitalized	(19)	61	10	90
Net periodic benefit cost (credit)	(261)	(82)	(150)	533
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,600	$ 1,800	$ 3,300	$ 3,600