Derivative Instruments--Derivative Settlements Outstanding (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative settlements receivable included in accounts receivable	$ 4,616	$ 8,013
Derivative settlements payable included in accounts payable and accrued liabilities	$ 377	$ 41