Change in benefit obligations and plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 475,337	$ 402,772
Service cost	86	129	$ 155
Interest cost	17,141	17,682	16,249
Plan participants' contributions	0	0
Actuarial (gain) loss	(24,875)	80,520
Benefits paid	(24,729)	(25,766)
Benefit obligation at end of year	442,960	475,337	402,772
Change in net plan assets:
Fair value of plan assets at beginning of year	354,363	334,844
Actual gain (loss) on plan assets	(10,879)	24,500
Employer contribution	13,912	20,785
Plan participants' contributions	0	0
Benefits paid	(24,729)	(25,766)
Fair value of net plan assets at end of year	332,667	354,363	334,844
Funded status - under	(110,293)	(120,974)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	0	0
Other accrued liabilities (current)	0	0
Other liabilities (noncurrent)	(110,293)	(120,974)
Net amount recognized	(110,293)	(120,974)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	208,671	207,430
Prior service cost (credit)	0	294
Total	208,671	207,724
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	99,012	81,726
Service cost	1,816	1,518	1,675
Interest cost	3,607	3,521	3,215
Plan participants' contributions	1,408	1,399
Actuarial (gain) loss	(5,873)	18,024
Benefits paid	(7,236)	(7,176)
Benefit obligation at end of year	92,734	99,012	81,726
Change in net plan assets:
Fair value of plan assets at beginning of year	87,586	84,543
Actual gain (loss) on plan assets	258	7,527
Employer contribution	577	1,293
Plan participants' contributions	1,408	1,399
Benefits paid	(7,236)	(7,176)
Fair value of net plan assets at end of year	82,593	87,586	$ 84,543
Funded status - under	(10,141)	(11,426)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	5,095	4,345
Other accrued liabilities (current)	(421)	(322)
Other liabilities (noncurrent)	(14,815)	(15,449)
Net amount recognized	(10,141)	(11,426)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	22,484	25,779
Prior service cost (credit)	(14,374)	(15,744)
Total	$ 8,110	$ 10,035

No definition available.

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- Definition

The pretax total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.