Components of net periodic benefit cost (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 86	$ 129
Interest cost	17,218	17,141	17,682
Expected return on assets	(20,924)	(22,254)	(21,218)
Amortization of prior service cost (credit)	0	36	71
Recognized net actuarial loss	6,215	7,016	4,869
Curtailment loss	0	258	0
Net periodic benefit cost (credit), including amount capitalized	2,509	2,283	1,533
Less amount capitalized	381	316	388
Net periodic benefit cost (credit)	2,128	1,967	1,145
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	(3,789)	8,257	77,238
Amortization of actuarial loss	(6,215)	(7,016)	(4,869)
Amortization of prior service (cost) credit	0	(294)	(71)
Total recognized in accumulated other comprehensive (income) loss	(10,004)	947	72,298
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	(7,876)	2,914	73,443
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,647	1,816	1,518
Interest cost	3,688	3,607	3,521
Expected return on assets	(4,533)	(4,795)	(4,617)
Amortization of prior service cost (credit)	(1,371)	(1,371)	(1,393)
Recognized net actuarial loss	1,491	1,960	649
Curtailment loss	0	0	0
Net periodic benefit cost (credit), including amount capitalized	922	1,217	(322)
Less amount capitalized	(52)	120	(21)
Net periodic benefit cost (credit)	974	1,097	(301)
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	(3,523)	(1,336)	15,114
Amortization of actuarial loss	(1,491)	(1,960)	(649)
Amortization of prior service (cost) credit	1,371	1,371	1,393
Total recognized in accumulated other comprehensive (income) loss	(3,643)	(1,925)	15,858
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	$ (2,669)	$ (828)	$ 15,557