Fair value measurements - Fair value of other financial instruments not recorded at fair value (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amount - Long-term debt, including current portion	$ 551,725	$ 543,567
Variable Interest Entity, Primary Beneficiary
Estimated fair values of financial instruments (other than derivative financial instruments)
Variable Interest Entity, Carrying Amount Of Long-term Debt Consolidated VIE Including Current Portion	95,700	96,150
Level 3
Estimated fair values of financial instruments (other than derivative financial instruments)
Fair value - Long-term debt, including current portion	551,698	562,588
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	$ 96,801	$ 97,775