Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Offsetting Assets [Line Items]
Gross amounts of recognized, asset	$ 11,818	[1]	$ 18,518	[1]
Gross amount offset in the balance sheet	0		0
Net amounts presented in the balance sheet, asset	11,818		18,518
Gross amounts not offset in the balance sheet, financial instruments, asset	1,739		(12,051)
Gross amounts not offset in the balance sheet, cash collateral pledged	0		0
Net amount, asset	13,557		6,467
Gross amounts of recognized, liability	(74,167)	[1]	(30,502)	[1]
Gross amount offset in the balance sheet	0		0
Net amounts presented in the balance sheet, liability	(74,167)		(30,502)
Gross amounts not offset in the balance sheet, financial instruments, liability	(1,739)		12,051
Gross amounts not offset in the balance sheet, cash collateral received	4,260		0
Net amount, liability	$ (71,646)		$ (18,451)

[1] Amounts include all derivative instruments, irrespective of whether there is a legally enforceable master netting arrangement in place.