Derivative Instruments - Balance sheet offsetting (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative assets - gross amounts recognized in the balance sheet	$ 136,659	$ 27,487
Derivative assets - gross amounts offset in the balance sheet	696	1,342
Derivative assets - net amounts in the balance sheet	135,963	26,145
Derivative assets - financial instruments not offset in the balance sheet	0	0
Derivative assets - cash collateral not offset in the balance sheet	78,580	0
Derivative assets - net amount	57,383us-gaap_DerivativeAssets	26,145us-gaap_DerivativeAssets
Derivative liabilities - gross amounts recognized in the balance sheet	31,184	105,485
Derivative liabilities - gross amounts offset in the balance sheet	696	1,342
Derivative liabilities - net amounts in the balance sheet	30,488	104,143
Derivative liabilities - financial instruments not offset in the balance sheet	0	0
Derivative liabilities - cash collateral not offset in the balance sheet	0	19,847
Derivative liabilities - net amount	$ 30,488us-gaap_DerivativeLiabilities	$ 84,296us-gaap_DerivativeLiabilities

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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Fair value, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Amount of obligation to return securities collateral under master netting arrangements that have not been offset against derivative assets.

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

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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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Fair value, after offset of derivative asset, of financial liability or 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value, after the effects of master netting arrangements, of a financial liability or 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. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or 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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