Repurchase Agreements Repurchase Agreements By Maturity Period (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	$ 4,071,392	$ 3,564,128
Deferred Costs	(451)	(659)
Maturity up to 30 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	1,677,943	622,957
Maturity 31 to 60 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	1,121,607	1,263,105
Matuity 61 to 90 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	353,523	298,660
Maturity over 90 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	918,770	1,380,065
Repurchase Agreements [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	$ 4,071,843	$ 3,564,787