Federal Funds Purchased and Securities Sold Under Agreements to Repurchase (Schedule of Select Financial Data Federal Funds Purchased and Securities Sold Under Agreements to Repurchase) (Details) (Federal Funds Purchased and Securities Sold under Agreements to Repurchase, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2013
Federal Funds Purchased and Securities Sold under Agreements to Repurchase
Average balance during the year	$ 1,084,532	$ 949,068	$ 949,756
Weighted-average annual interest rate during the year	0.09%	0.13%		0.12%
Maximum month-end balance	$ 1,289,460	$ 1,123,795	$ 1,104,525