Change in benefit obligations and plan assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 435,618	$ 388,589
Service cost	1,078	2,252	2,889
Interest cost	17,598	19,500	19,761
Plan participants' contributions	0	0
Amendments	0	0
Actuarial loss	30,939	62,722
Curtailment gain	0	(13,939)
Benefits paid	(26,122)	(23,506)
Benefit obligation at end of year	459,111	435,618	388,589
Change in net plan assets:
Fair value of plan assets at beginning of year	278,000	277,598
Actual gain (loss) on plan assets	34,493	(4,718)
Employer contribution	22,813	28,626
Plan participants' contributions	0	0
Benefits paid	(26,122)	(23,506)
Fair value of net plan assets at end of year	309,184	278,000	277,598
Funded status - under	(149,927)	(157,618)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other accrued liabilities (current)	0	0
Other liabilities (noncurrent)	(149,927)	(157,618)
Net amount recognized	(149,927)	(157,618)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	202,406	189,494
Prior service cost (credit)	437	(632)
Transition obligation	0	0
Total	202,843	188,862
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	110,689	91,286
Service cost	1,747	1,443	1,357
Interest cost	4,166	4,700	4,817
Plan participants' contributions	2,688	2,644
Amendments	(11,418)	0
Actuarial loss	3,469	17,940
Curtailment gain	0	0
Benefits paid	(7,983)	(7,324)
Benefit obligation at end of year	103,358	110,689	91,286
Change in net plan assets:
Fair value of plan assets at beginning of year	68,085	70,610
Actual gain (loss) on plan assets	6,497	(872)
Employer contribution	5,074	3,027
Plan participants' contributions	2,688	2,644
Benefits paid	(7,983)	(7,324)
Fair value of net plan assets at end of year	74,361	68,085	70,610
Funded status - under	(28,997)	(42,604)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Other accrued liabilities (current)	(655)	(550)
Other liabilities (noncurrent)	(28,342)	(42,054)
Net amount recognized	(28,997)	(42,604)
Amounts recognized in accumulated other comprehensive (income) loss consist of:
Actuarial loss	43,589	43,861
Prior service cost (credit)	(18,594)	(8,615)
Transition obligation	0	2,128
Total	$ 24,995	$ 37,374