Summary of Offsetting Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives instrument
Gross fair value of recognized assets	$ 25,837	$ 26,647
Net fair value presented in balance sheets	25,837	26,647
Gross fair value not offset in balance sheets
Financial Instruments	25,837	(26,647)
Short-term repurchase agreement
Gross fair value of recognized assets	750,000	4,138
Net fair value presented in balance sheets	750,000	4,138
Gross fair value not offset in balance sheets
Financial Instruments	750,000	(4,138)
Total financial instrument
Gross fair value of recognized assets	775,837	30,785
Net fair value presented in balance sheets	775,837	30,785
Gross fair value not offset in balance sheets
Financial Instruments	775,837	(30,785)
Derivatives instrument
Gross fair value of recognized assets	103,436	87,093
Net fair value presented in balance sheets	103,436	87,093
Gross fair value not offset in balance sheets
Financial Instruments	25,837	26,647
Cash collateral received	74,780	54,400
Net fair value	$ 2,819	$ 6,046

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Amount of obligation to return financial securities collateral under master netting arrangements that have not been offset against securities purchased under agreement to resell.

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Amount, before effects of master netting arrangements, of funds outstanding loaned in the form of a security resale agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price.

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