Fair Value Measurements (Details) (USD $)	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Level 2 [Member] Portion at Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2012 Level 2 [Member] Portion at Fair Value, Fair Value Disclosure [Member]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset			$ 20,577,000	$ 36,452,000
Derivative Asset, Fair Value, Gross Liability			(7,801,000)	(7,291,000)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			12,776,000	29,161,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability			11,686,000	7,291,000
Derivative Liability, Fair Value, Gross Asset			7,801,000	7,291,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral			3,885,000	0
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset (Liability)			8,891,000	29,161,000
Derivative Asset (Liability), Fair Value, Gross Liability (Asset)			0	0
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net			8,891,000	29,161,000
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0