Variable interest entities - Carrying amounts of consolidated assets and liabilities of Charleston Center LLC (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Variable Interest Entity [Line Items]
Current assets	$ 271,751	$ 281,796
Property, plant and equipment	191,471	187,854
Goodwill	148,602	156,916
Other assets	54,327	53,998
Total assets	1,819,920	1,879,866
Current liabilities	152,532	209,479
Long-term accrued interest on subordinated debt	15,490	15,340
Deferred income taxes of consolidated variable interest entities	61,034	60,892
Total liabilities	978,503	969,221
Current portion of long-term debt of consolidated variable interest entities	1,772	1,805
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Third-party debt, including $1,772 and $1,805 current portion	95,700	96,150
Charleston Center LLC
Variable Interest Entity [Line Items]
Property, plant and equipment	191,471	187,854
Charleston Center LLC | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Current assets	21,903	10,517
Property, plant and equipment	191,471	187,854
Goodwill	40,395	40,395
Other assets	1,690	1,895
Total assets	255,459	240,661
Current liabilities	6,665	6,722
Third-party debt, including $1,772 and $1,805 current portion	95,700	96,150
Long-term accrued interest on subordinated debt	15,490	15,340
Deferred income taxes of consolidated variable interest entities	61,034	60,892
Total liabilities	178,889	179,104
Net assets (before amounts payable to OEH of $94,039 and $92,692)	76,570	61,557
Amounts payable to OEH	94,039	92,692
Current portion of long-term debt of consolidated variable interest entities	$ 1,772	$ 1,805