Repurchase Agreements Repurchase Agreements, Term to Original Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 3,055,069	$ 3,013,110
Less than 30 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	602,224	250,635
30 to 90 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	2,224,106	617,399
91 to 180 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	178,242	904,830
181 to 364 days [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	1,030,569
1 year or longer [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 50,497	$ 209,677