Quarterly report pursuant to Section 13 or 15(d)

Repurchase Agreements Schedule of Underlying Assets of Repurchase Agreements when Amount of Repurchase Agreements Exceeds 10 Percent of Assets (Details)

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Repurchase Agreements Schedule of Underlying Assets of Repurchase Agreements when Amount of Repurchase Agreements Exceeds 10 Percent of Assets (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
US Treasury Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value 1,002,414 1,002,062
Loans Held-for-sale, Mortgages [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value 25,909 52,529
Cash and Cash Equivalents [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value 15,000 10,000
Restricted Cash and Cash Equivalents [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value 129,148 84,307
Due From Correspondent Brokers [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value 13,351 36,917
Derivative Financial Instruments, Assets [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Market Value $ 291,295 $ 291,054