Employee benefit plans (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Defined Benefit Plan Disclosure
Current year contributions			$ 1,809,000	$ 1,541,000
Pension benefits:
Defined Benefit Plan Disclosure
Service costs	289,000	241,000	565,000	481,000
Interest costs	754,000	687,000	1,507,000	1,373,000
Expected return on plan assets	(768,000)	(659,000)	(1,534,000)	(1,317,000)
Recognized net actuarial loss	162,000	344,000	326,000	688,000
Net periodic cost (benefit)	437,000	613,000	864,000	1,225,000
Postretirement health care and life insurance plans:
Defined Benefit Plan Disclosure
Interest costs	20,000	19,000	40,000	39,000
Amortization of prior service cost	(454,000)	(454,000)	(908,000)	(908,000)
Recognized net actuarial loss	450,000	456,000	900,000	912,000
Net periodic cost (benefit)	16,000	21,000	32,000	43,000
United States
Defined Benefit Plan Disclosure
Total expected current year contributions			3,337,000
International
Defined Benefit Plan Disclosure
Total expected current year contributions			$ 1,555,000