Derivatives Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Offsetting (Details) [Abstract]
Gross amounts of recognized, asset	$ 21,521	[1]	$ 44,508	[1]
Gross amount offset in the balance sheet	0		0
Net amounts of asset/ liabilities presented in the balance sheet, asset	21,521		44,508
Gross amounts not offset in the balance sheet, financial instruments, asset	(766)		(2)
Gross amounts not offset in the balance sheet, cash collateral received/ pledged	(8,536)		0
Net amount, asset	12,219		44,506
Gross amounts of recognized, liability	(27,977)	[1]	(14,010)	[1]
Gross amount offset in the balance sheet	0		0
Net amounts presented in the balance sheet, liability	(27,977)		(14,010)
Gross amounts not offset in the balance sheet, financial instruments, liability	766		2
Gross amounts not offset in the balance sheet, cash collateral received	14,858		4,341
Net amount, liability	$ (12,353)		$ (9,667)

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