Change in benefit obligations and plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 442,960	$ 475,337
Service cost	0	86	$ 129
Interest cost	17,218	17,141	17,682
Plan participants' contributions	0	0
Actuarial (gain) loss	1,882	(24,875)
Benefits paid	(25,753)	(24,729)
Benefit obligation at end of year	436,307	442,960	475,337
Change in net plan assets:
Fair value of plan assets at beginning of year	332,667	354,363
Actual gain (loss) on plan assets	26,595	(10,879)
Employer contribution	0	13,912
Plan participants' contributions	0	0
Benefits paid	(25,753)	(24,729)
Fair value of net plan assets at end of year	333,509	332,667	354,363
Funded status - under	(102,798)	(110,293)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	0	0
Other accrued liabilities (current)	0	0
Other liabilities (noncurrent)	(102,798)	(110,293)
Net amount recognized	(102,798)	(110,293)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	198,668	208,671
Prior service cost (credit)	0	0
Total	198,668	208,671
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	92,734	99,012
Service cost	1,647	1,816	1,518
Interest cost	3,688	3,607	3,521
Plan participants' contributions	1,405	1,408
Actuarial (gain) loss	(3,872)	(5,873)
Benefits paid	(6,298)	(7,236)
Benefit obligation at end of year	89,304	92,734	99,012
Change in net plan assets:
Fair value of plan assets at beginning of year	82,593	87,586
Actual gain (loss) on plan assets	4,184	258
Employer contribution	962	577
Plan participants' contributions	1,405	1,408
Benefits paid	(6,298)	(7,236)
Fair value of net plan assets at end of year	82,846	82,593	$ 87,586
Funded status - under	(6,458)	(10,141)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	13,131	5,095
Other accrued liabilities (current)	(538)	(421)
Other liabilities (noncurrent)	(19,051)	(14,815)
Net amount recognized	(6,458)	(10,141)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	17,470	22,484
Prior service cost (credit)	(13,003)	(14,374)
Total	$ 4,467	$ 8,110

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.