Balance Sheet Offsetting (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Balance Sheet Offsetting [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 16,145	$ 16,740
Derivative Asset, Amount Offset	0	0
Derivative Asset, Fair Value, Net	16,145	16,740
Derivative Liability, Fair Value, Gross Liability	16,794	17,718
Derivative Liability, Amount Offset	0	0
Derivative Liability, Fair Value, Net	16,794	17,718
Derivatives Subject to Master Netting Agreement [Member]
Balance Sheet Offsetting [Line Items]
Derivative Asset, Fair Value, Gross Asset	15,752	16,475
Derivative Asset, Amount Offset	0	0
Derivative Asset, Fair Value, Net	15,752	16,475
Derivative Asset, Fair Value, Gross Liability	(1,180)	(603)
derivatve asset, fair value of collateral	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	14,572	15,872
Derivative Liability, Fair Value, Gross Liability	16,105	17,315
Derivative Liability, Amount Offset	0	0
Derivative Liability, Fair Value, Net	16,105	17,315
Derivative Liability, Fair Value, Gross Asset	(1,180)	(603)
Derivative Liability, fair value of collateral	(13,558)	(16,017)
Derivative Liability, Fair Value, Amount Offset Against Collateral	1,367	695
Derivatives Not Subject to Master Netting Agreement [Member]
Balance Sheet Offsetting [Line Items]
Derivative Asset, Fair Value, Gross Asset	393	265
Derivative Asset, Amount Offset	0	0
Derivative Asset, Fair Value, Net	393	265
Derivative Liability, Fair Value, Gross Liability	689	403
Derivative Liability, Amount Offset	0	0
Derivative Liability, Fair Value, Net	689	403
Resale agreement [Member]
Balance Sheet Offsetting [Line Items]
Securities purchased under agreements to resell, gross	1,500,000	1,500,000
Securities purchased under agreements to resell, liability	(300,000)	(300,000)
Securities purchased under agreements to resell, net	1,200,000	1,200,000
Securities Purchased under Agreements to Resell, Not Subject to Master Netting Arrangement	0	0
Securities Purchased under Agreements to Resell, Collateral, Obligation to Return Securities	(1,200,000)	(1,200,000)
Securities Purchased under Agreements to Resell, Amount Offset Against Collateral	0	0
Repurchase agreement [Member]
Balance Sheet Offsetting [Line Items]
Securities Sold under Agreements to Repurchase, Gross	1,390,108	1,359,040
Securities Sold under Agreements to Repurchase, Asset	(300,000)	(300,000)
Securities sold under agreements to repurchase, net	1,090,108	1,059,040
Securities Sold under Agreements to Repurchase, Not Subject to Master Netting Arrangement	0	0
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	(1,090,108)	(1,059,040)
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	$ 0	$ 0