Short-Term Borrowings (Details) (Securities Sold under Agreements to Repurchase [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Securities Sold under Agreements to Repurchase [Member]
Securities sold under agreements to repurchase [Line Items]
Ending balance	$ 310,463	$ 303,786	$ 242,703
Weighted average interest rate	0.14%	0.12%	0.22%
Average amount outstanding during year	312,729	287,176	249,725
Weighted average interest rate during year	0.14%	0.18%	0.26%
Maximum balance outstanding at any month-end	$ 335,193	$ 313,626	$ 259,534