Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Offsetting [Abstract]
Gross amounts of recognized, asset	$ 19,755	[1]	$ 21,521	[1]
Gross amount offset in the balance sheet	0		0
Net amounts of asset/ liabilities presented in the balance sheet, asset	19,755		21,521
Gross amounts not offset in the balance sheet, financial instruments, asset	(385)		(766)
Gross amounts not offset in the balance sheet, cash collateral received/ pledged	(17,112)		(8,536)
Net amount, asset	2,258		12,219
Gross amounts of recognized, liability	(52,540)	[1]	(27,977)	[1]
Gross amount offset in the balance sheet	0		0
Net amounts presented in the balance sheet, liability	(52,540)		(27,977)
Gross amounts not offset in the balance sheet, financial instruments, liability	385		766
Gross amounts not offset in the balance sheet, cash collateral received	45,734		14,858
Net amount, liability	$ (6,421)		$ (12,353)

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