Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 32	$ 86	$ 96
Interest cost	4,285	4,420	12,855	13,265
Expected return on assets	(5,563)	(5,304)	(16,689)	(15,913)
Amortization of prior service cost (credit)	0	18	36	54
Amortization of net actuarial loss	1,734	1,217	5,282	3,651
Curtailment loss			258	0
Net periodic benefit cost (credit), including amount capitalized	456	383	1,828	1,153
Less amount capitalized	90	27	219	195
Net periodic benefit cost	366	356	1,609	958
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	454	379	1,362	1,138
Interest cost	902	919	2,705	2,701
Expected return on assets	(1,199)	(1,154)	(3,597)	(3,463)
Amortization of prior service cost (credit)	(343)	(348)	(1,028)	(1,044)
Amortization of net actuarial loss	511	162	1,525	486
Curtailment loss			0	0
Net periodic benefit cost (credit), including amount capitalized	325	(42)	967	(182)
Less amount capitalized	36	(65)	98	(55)
Net periodic benefit cost	289	23	869	(127)
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost	$ 1,700	$ 1,700	$ 5,300	$ 5,000