Offsetting Assets and Liabilities Offsettting Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Offsetting Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 21,192	$ 42,537
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	21,192	42,537
Derivative, Collateral, Right to Reclaim Securities	(20,420)	(42,499)
Derivative, Collateral, Right to Reclaim Cash	(38)	(38)
Derivative Liability, Fair Value, Amount Offset Against Collateral	734	0
Securities Sold under Agreements to Repurchase, Gross	4,071,392	3,564,128
Securities Sold under Agreements to Repurchase, Asset	0	0
Securities Sold under Agreements to Repurchase	4,071,392	3,564,128
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	(4,071,392)	(3,564,128)
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash	0	0
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	0	0
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Gross	4,092,584	3,606,665
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Asset	0	0
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned	4,092,584	3,606,665
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Collateral, Right to Reclaim Securities	(4,091,812)	(3,606,627)
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Collateral, Right to Reclaim Cash	(38)	(38)
Derivative Liability, Securities Sold under Agreements to Resell, Securities Loaned, Amount Offset Against Collateral	$ 734	$ 0