Quarterly report pursuant to Section 13 or 15(d)

Available-for-Sale Securities, at Fair Value Available-for-Sale Securities Sold Under Agreements to Repurchase (Details)

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Available-for-Sale Securities, at Fair Value Available-for-Sale Securities Sold Under Agreements to Repurchase (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value $ 15,096,848 $ 14,287,224
Available-for-sale Securities [Member]
   
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value $ 13,619,731 $ 12,810,355