Short-Term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Balance at year-end	$ 719,123	$ 681,703
FHLB, unused lines of credit available for short-term financing	2,000,000	1,900,000
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Balance at year-end	60,000	50,000	$ 99,500
Average during the year	54,162	65,257	97,424
Maximum month end balance	$ 80,000	$ 85,000	$ 159,000
Weighted average rate during the year	2.16%	0.98%	0.36%
Weighted average rate at December 31	2.41%	1.19%	0.51%
Securities Sold Under Repurchase Agreements [Member]
Short-term Debt [Line Items]
Balance at year-end	$ 182,123	$ 173,703	$ 167,981
Average during the year	175,539	168,821	162,201
Maximum month end balance	$ 190,326	$ 189,875	$ 178,326
Weighted average rate during the year	0.07%	0.06%	0.06%
Weighted average rate at December 31	0.07%	0.07%	0.06%
Other Short-Term Borrowings [Member]
Short-term Debt [Line Items]
Balance at year-end	$ 477,000	$ 458,000	$ 175,000
Average during the year	460,334	263,575	80,260
Maximum month end balance	$ 591,000	$ 424,000	$ 175,000
Weighted average rate during the year	1.02%	0.59%	0.42%
Weighted average rate at December 31	1.18%	0.70%	0.56%