Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of securities sold under agreements to repurchase
Repurchase Amount	$ 289,508	$ 258,643
Weighted Average Fixed Rate	0.32%	0.42%
Amortized Cost of Underlying Assets	291,825	261,979
Fair Value of Underlying Assets	298,264	267,508
Overnight [Member]
Summary of securities sold under agreements to repurchase
Repurchase Amount	285,349	257,802
Weighted Average Fixed Rate	0.32%	0.42%
Amortized Cost of Underlying Assets	287,597	260,124
Fair Value of Underlying Assets	293,958	265,592
Maturing within 30 days [Member]
Summary of securities sold under agreements to repurchase
Repurchase Amount	4,159	841
Weighted Average Fixed Rate	0.50%	1.00%
Amortized Cost of Underlying Assets	4,228	1,855
Fair Value of Underlying Assets	$ 4,306	$ 1,916