Fair value of financial instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amounts - Long-term debt, including current portion, excluding obligations under capital leases	$ 543,567	$ 521,494
Variable interest entity, primary beneficiary
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amounts - Long-term debt, including current portion, held by consolidated variable interest entities	96,150	97,945
Level 3
Estimated fair values of financial instruments (other than derivative financial instruments)
Fair value - Long-term debt, including current portion, excluding obligations under capital leases	562,588	533,783
Level 3 | Variable interest entity, primary beneficiary
Estimated fair values of financial instruments (other than derivative financial instruments)
Fair value - Long-term debt, including current portion, excluding obligations under capital leases	$ 97,775	$ 99,656