Short-Term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term borrowings [Abstract]
Balance at year-end	$ 871,696	$ 719,123
FHLB, unused lines of credit available for short-term financing	1,900,000	2,000,000
Federal Funds Purchased [Member]
Short-term borrowings [Abstract]
Balance at year-end	80,000	60,000	$ 50,000
Average during the year	66,839	54,162	65,257
Maximum month end balance	$ 80,000	$ 80,000	$ 85,000
Weighted average rate during the year	3.43%	2.16%	0.98%
Weighted average rate at year-end	4.25%	2.41%	1.19%
Securities Sold Under Repurchase Agreements [Member]
Short-term borrowings [Abstract]
Balance at year-end	$ 160,696	$ 182,123	$ 173,703
Average during the year	146,135	175,539	168,821
Maximum month end balance	$ 170,350	$ 190,326	$ 189,875
Weighted average rate during the year	0.16%	0.07%	0.06%
Weighted average rate at year-end	0.38%	0.07%	0.07%
Other Short-Term Borrowings [Member]
Short-term borrowings [Abstract]
Balance at year-end	$ 631,000	$ 477,000	$ 458,000
Average during the year	514,662	460,334	263,575
Maximum month end balance	$ 653,000	$ 591,000	$ 424,000
Weighted average rate during the year	1.56%	1.02%	0.59%
Weighted average rate at year-end	1.82%	1.18%	0.70%