Short-Term Borrowings (Short-Term Borrowings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,703,890	$ 1,225,406
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	140,754	2,275
Average amount outstanding during period	27,063	14,052
Maximum amount at any month end during period	$ 140,754	$ 59,475
Weighted-average interest at period end	1.00%	0.38%
Weighted-average interest rate during period	1.37%	0.50%
Securities Sold Under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	$ 430,569	$ 358,131
Average amount outstanding during period	501,719	454,571
Maximum amount at any month end during period	$ 587,569	$ 579,099
Weighted-average interest at period end	0.17%	0.04%
Weighted-average interest rate during period	0.12%	0.03%
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,132,567	$ 865,000
Average amount outstanding during period	1,478,114	943,570
Maximum amount at any month end during period	$ 2,061,652	$ 1,175,000
Weighted-average interest at period end	1.35%	0.54%
Weighted-average interest rate during period	1.00%	0.41%