Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Repurchase Agreements
Repurchase Amount	$ 313,394	$ 289,508
Weighted Average Fixed Rate	0.28%	0.32%
Amortized Cost of Underlying Assets	315,358	291,825
Fair Value of Underlying Assets	324,157	298,264
Overnight
Repurchase Agreements
Repurchase Amount	303,709	285,349
Weighted Average Fixed Rate	0.28%	0.32%
Amortized Cost of Underlying Assets	304,263	287,597
Fair Value of Underlying Assets	312,856	293,958
Maturing within 30 days
Repurchase Agreements
Repurchase Amount	9,685	4,159
Weighted Average Fixed Rate	0.32%	0.50%
Amortized Cost of Underlying Assets	11,095	4,228
Fair Value of Underlying Assets	$ 11,301	$ 4,306