Postretirement benefits (obligations and funded status) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Change in benefit obligation
Interest cost	$ 3,652	$ 6,061	$ 6,833
Amounts recognized in balance sheets
Other non-current assets	24,981	4,993
Postretirement benefit plan
Postretirement benefits [Line Items]
Actuarial gain due to EGWP		15,600
Amortization period for actuarial gains and losses generated in current year		17 years
Plan amendment		5,063
Recognition period for prior service credit generated in current year		17 years
Change in benefit obligation
Benefit obligation, beginning of year	117,230	146,125
Interest cost	3,535	5,913
Net actuarial (gain) loss	(2,666)	(19,917)
Benefits paid from plan assets and company funds	(10,386)	(10,679)
Plan amendment		5,063
Medicare Part D reimbursements	854	851
Benefit obligation, end of year	108,567	117,230
Change in plan assets
Fair value of plan assets, beginning of year	122,223	100,622
Actual gain on plan assets	18,886	14,001
Company contributions	0	7,600
Benefits paid	(7,561)
Fair value of plan assets, end of year	133,548	122,223
Funded status	24,981	4,993
Designation of plan assets
Percentage of participants who retired after 1986	83.00%
Amounts recognized in balance sheets
Other non-current assets	24,981	4,993
Accrued liabilities	0	0
Other non-current liabilities	0	0
Amounts recognized in accumulated other comprehensive loss
Unrecognized prior service credit	21,284	22,705
Unrecognized net actuarial loss	(70,350)	(88,548)
Tax effect	14,192	20,540
Amount recognized in accumulated other comprehensive loss, net of tax	(34,874)	(45,303)
Weighted-average amortization period, prior service credit	21 years
Amortization period net actuarial loss	17 years
Amounts in accumulated other comprehensive loss which are expected to be recognized in the next 12 months
Prior service credit	(1,421)
Net actuarial loss	3,418
Total	1,997
Postretirement benefit plan | Retiree medical benefits for post-1986 retirees
Change in plan assets
Fair value of plan assets, end of year	124,730	114,037
Postretirement benefit plan | Post-2012 prescription drug benefits for pre-1986 retirees
Change in plan assets
Fair value of plan assets, end of year	8,818	8,186
Pension plan
Change in benefit obligation
Benefit obligation, beginning of year	3,872	3,680
Interest cost	117	148
Net actuarial (gain) loss	(237)	368
Benefits paid from plan assets and company funds	(324)	(324)
Benefit obligation, end of year	3,428	3,872
Change in plan assets
Funded status	(3,428)	(3,872)
Amounts recognized in balance sheets
Other non-current assets	0	0
Accrued liabilities	324	324
Other non-current liabilities	$ 3,104	$ 3,548