Derivatives (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Period over which gain included in AOCI will be transferred to earnings, months	12 months
Expected earnings transfer	$ (3,838)
Expected derivative maturity date	Dec. 31, 2013
Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated OCI gain (loss), net of tax	$ (3,838)	[1]	$ 13,503	[1]

[1] Net of taxes.