Accrued Liabilities and Deferred Credits (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Payables and Accruals [Abstract]
Accrued compensation	$ 2,442	$ 3,529
Deferred revenue	30,286	27,962
Environmental and insurance liabilities	1,541	1,621
Other accrued liabilities	16,354	9,240
Total accrued liabilities and deferred credits	$ 50,623	$ 42,352