Components of net periodic benefit cost (Details 3) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 86	$ 129	$ 155
Interest cost	17,141	17,682	16,249
Expected return on assets	(22,254)	(21,218)	(19,917)
Amortization of prior service cost (credit)	36	71	71
Recognized net actuarial loss	7,016	4,869	7,173
Curtailment loss	258	0	0
Net periodic benefit cost (credit), including amount capitalized	2,283	1,533	3,731
Less amount capitalized	316	388	727
Net periodic benefit cost (credit)	1,967	1,145	3,004
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	8,257	77,238	(60,173)
Amortization of actuarial loss	(7,016)	(4,869)	(7,173)
Amortization of prior service (cost) credit	(294)	(71)	(71)
Total recognized in accumulated other comprehensive (income) loss	947	72,298	(67,417)
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	2,914	73,443	(64,413)
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,816	1,518	1,675
Interest cost	3,607	3,521	3,215
Expected return on assets	(4,795)	(4,617)	(4,343)
Amortization of prior service cost (credit)	(1,371)	(1,393)	(1,457)
Recognized net actuarial loss	1,960	649	1,814
Curtailment loss	0	0	0
Net periodic benefit cost (credit), including amount capitalized	1,217	(322)	904
Less amount capitalized	120	(21)	164
Net periodic benefit cost (credit)	1,097	(301)	740
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive (income) loss:
Net (gain) loss	(1,336)	15,114	(30,461)
Amortization of actuarial loss	(1,960)	(649)	(1,814)
Amortization of prior service (cost) credit	1,371	1,393	1,457
Total recognized in accumulated other comprehensive (income) loss	(1,925)	15,858	(30,818)
Total recognized in net periodic benefit cost (credit) and accumulated other comprehensive (income) loss	$ (828)	$ 15,557	$ (30,078)

The total amount of net periodic benefit cost for defined benefit plans capitalized for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments).

No definition available.