Derivative Instruments - Balance sheet offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative assets:
Gross amounts recognized in balance sheet	$ 108,745	$ 24,842
Gross amounts offset in balance sheet	680	1,327
Net amounts in the balance sheet	108,065	23,515
Gross amounts not offset in the balance sheet - financial instruments	0	0
Gross amounts not offset in the balance sheet - cash collateral	46,720	0
Net amounts	61,345	23,515
Derivative liabilities:
Gross amounts recognized in balance sheet	61,394	66,626
Gross amounts offset in balance sheet	680	1,327
Net amounts in the balance sheet	60,714	65,299
Gross amounts not offset in the balance sheet - financial instruments	0	0
Gross amounts not offset in the balance sheet - cash collateral	0	12,130
Net amounts	$ 60,714	$ 53,169

Fair value of liability associated with derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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-Publisher FASB

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The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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The amount as of the balance sheet date of the fair value of derivative assets that in accordance with the entity's accounting policy was not offset against an obligation to return cash collateral under a master netting arrangement.

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The amount as of the balance sheet date of the fair value of derivative assets that in accordance with the entity's accounting policy was offset against an obligation to return cash collateral under a master netting arrangement.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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The amount as of the balance sheet date of the fair value of derivative liabilities that in accordance with the entity's accounting policy was not offset against the right to reclaim cash collateral under a master netting arrangement.

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The amount as of the balance sheet date of the fair value of derivative liabilities that in accordance with the entity's accounting policy were offset against the right to reclaim cash collateral under a master netting arrangement.

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The fair value of collateral received by the entity associated with securities borrowed that must be returned if the counterparty fulfills its obligations. It appears on the balance sheet as a liability and has a corresponding asset that appears on the balance sheet called Securities Received as Collateral.

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Amount of right to receive securities collateral under master netting arrangements that have not been offset against derivative liabilities.

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