Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 40	$ 33
Interest cost	4,364	4,440
Expected return on assets	(5,373)	(5,125)
Amortization of prior service cost (credit)	18	18
Amortization of net actuarial loss	1,735	1,313
Net periodic benefit cost, including amount capitalized	784	679
Less amount capitalized	76	95
Net periodic benefit cost	708	584
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	483	379
Interest cost	914	858
Expected return on assets	(1,175)	(1,067)
Amortization of prior service cost (credit)	(342)	(348)
Amortization of net actuarial loss	461	318
Net periodic benefit cost, including amount capitalized	341	140
Less amount capitalized	29	29
Net periodic benefit cost	312	111
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,700	$ 1,700