Change in benefit obligations and plan assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 402,772	$ 459,111
Service cost	129	155	1,078
Interest cost	17,682	16,249	17,598
Plan participants' contributions	0	0
Actuarial (gain) loss	80,520	(44,551)
Benefits paid	(25,766)	(28,192)
Benefit obligation at end of year	475,337	402,772	459,111
Change in net plan assets:
Fair value of plan assets at beginning of year	334,844	309,184
Actual gain on plan assets	24,500	35,539
Employer contribution	20,785	18,313
Plan participants' contributions	0	0
Benefits paid	(25,766)	(28,192)
Fair value of net plan assets at end of year	354,363	334,844	309,184
Funded status - (under) over	(120,974)	(67,928)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	0	0
Other accrued liabilities (current)	0	0
Other liabilities (noncurrent)	(120,974)	(67,928)
Net amount recognized	(120,974)	(67,928)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	207,430	135,061
Prior service cost (credit)	294	365
Total	207,724	135,426
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	81,726	103,358
Service cost	1,518	1,675	1,747
Interest cost	3,521	3,215	4,166
Plan participants' contributions	1,399	1,472
Actuarial (gain) loss	18,024	(20,985)
Benefits paid	(7,176)	(7,009)
Benefit obligation at end of year	99,012	81,726	103,358
Change in net plan assets:
Fair value of plan assets at beginning of year	84,543	74,361
Actual gain on plan assets	7,527	13,819
Employer contribution	1,293	1,900
Plan participants' contributions	1,399	1,472
Benefits paid	(7,176)	(7,009)
Fair value of net plan assets at end of year	87,586	84,543	74,361
Funded status - (under) over	(11,426)	2,817
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other assets (noncurrent)	4,345	9,679
Other accrued liabilities (current)	(322)	(381)
Other liabilities (noncurrent)	(15,449)	(6,481)
Net amount recognized	(11,426)	2,817
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	25,779	11,314
Prior service cost (credit)	(15,744)	(17,137)
Total	$ 10,035	$ (5,823)

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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.

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The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

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Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost.

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The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

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Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

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The aggregate net amount recognized in the balance sheet associated with the defined benefit plan(s). Will normally be the same as the Defined Benefit Plan, Funded Status of Plan, Total.

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The amount for overfunded plans recognized in the balance sheet as a noncurrent asset associated with a defined benefit pension plan or other postretirement defined benefit plan.

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1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

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The increase in the fair value of plan assets from contributions made by the employer.

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The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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For a classified balance sheet, the amount recognized in balance sheet as a current liability associated with an underfunded defined benefit plan.

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