Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets and liabilities measured at fair value on recurring basis
Equity investment	$ 10,471	$ 12,994
Fair value on recurring [Member]
Assets and liabilities measured at fair value on recurring basis
Equity investment	10,471	12,994
Derivative financial instruments - assets	51,594	72,319
Derivative financial instruments - liabilities	(8,423)	(9,834)
Assets and liabilities fair value	53,642	75,479
Fair value on recurring [Member] | Level 1 [Member]
Assets and liabilities measured at fair value on recurring basis
Equity investment	9,352	11,601
Assets and liabilities fair value	9,352	11,601
Fair value on recurring [Member] | Level 2 [Member]
Assets and liabilities measured at fair value on recurring basis
Equity investment	1,119	1,393
Derivative financial instruments - assets	51,594	72,319
Derivative financial instruments - liabilities	(8,423)	(9,834)
Assets and liabilities fair value	44,290	63,878
Fair value on recurring [Member] | Level 3 [Member]
Assets and liabilities measured at fair value on recurring basis
Equity investment
Derivative financial instruments - assets
Derivative financial instruments - liabilities
Assets and liabilities fair value