Derivative Instruments - Balance sheet offsetting (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative assets:
Gross amounts recognized in balance sheet	$ 30,579	$ 24,842
Gross amounts offset in balance sheet	2,600	1,327
Net amounts in the balance sheet	27,979	23,515
Gross amounts not offset in the balance sheet - financial instruments	0	0
Gross amounts not offset in the balance sheet - cash collateral	0	0
Derivative Asset	27,979	23,515
Derivative liabilities:
Gross amounts recognized in balance sheet	201,401	66,626
Gross amounts offset in balance sheet	2,600	1,327
Net amounts in the balance sheet	198,801	65,299
Gross amounts not offset in the balance sheet - financial instruments	0	0
Gross amounts not offset in the balance sheet - cash collateral	113,426	12,130
Derivative Liability	$ 85,375	$ 53,169

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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