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	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Average balance during the year	$ 1,138,307	$ 1,084,532	$ 949,068
Weighted-average annual interest rate during the year	0.10%	0.09%	0.13%
Maximum month-end balance	$ 1,519,250	$ 1,289,460	$ 1,123,795