Short-Term Borrowings (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Balance at year-end	$ 316,802,000	$ 456,042,000
FHLB, unused lines of credit available for short-term financing	958,000,000	497,000,000
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Balance at year-end	68,000,000	130,000,000	10,000,000
Average during the year	110,154,000	39,907,000	12,658,000
Maximum month end balance	183,000,000	130,000,000	60,000,000
Weighted average rate during the year (in hundredths)	0.29%	0.26%	0.27%
Weighted average rate at December 31 (in hundredths)	0.34%	0.18%	0.27%
Securities Sold under Repurchase Agreements [Member]
Short-term Debt [Line Items]
Balance at year-end	148,802,000	176,042,000	152,941,000
Average during the year	165,858,000	169,352,000	153,084,000
Maximum month end balance	182,861,000	185,871,000	165,977,000
Weighted average rate during the year (in hundredths)	0.06%	0.06%	0.10%
Weighted average rate at December 31 (in hundredths)	0.06%	0.05%	0.10%
Other Short-Term Borrowings [Member]
Short-term Debt [Line Items]
Balance at year-end	100,000,000	150,000,000	0
Average during the year	106,438,000	71,589,000	0
Maximum month end balance	$ 320,000,000	$ 210,000,000	$ 0
Weighted average rate during the year (in hundredths)	0.40%	0.43%	0.00%
Weighted average rate at December 31 (in hundredths)	0.36%	0.55%	0.00%