Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 33	$ 40
Interest cost	4,440	4,018
Expected return on assets	(5,125)	(5,083)
Amortization of prior service cost (credit)	18	18
Amortization of net actuarial loss	1,313	1,864
Net periodic benefit cost, including amount capitalized	679	857
Less amount capitalized	95	110
Net periodic benefit cost	584	747
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	379	504
Interest cost	858	940
Expected return on assets	(1,067)	(1,107)
Amortization of prior service cost (credit)	(348)	(364)
Amortization of net actuarial loss	318	671
Net periodic benefit cost, including amount capitalized	140	644
Less amount capitalized	29	29
Net periodic benefit cost	111	615
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,700	$ 1,900