Short-Term Borrowings (Short-Term Borrowings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,589,128	$ 1,703,890
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	425	140,754
Average amount outstanding during period	39,968	27,063
Maximum amount at any month end during period	$ 100,925	$ 140,754
Weighted-average interest at period end	2.00%	1.00%
Weighted-average interest rate during period	2.11%	1.37%
Securities Sold Under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	$ 428,599	$ 430,569
Average amount outstanding during period	456,000	501,719
Maximum amount at any month end during period	$ 500,345	$ 587,569
Weighted-average interest at period end	0.32%	0.17%
Weighted-average interest rate during period	0.23%	0.12%
FHLB Borrowings [Member]
Short-term Debt [Line Items]
Amount outstanding at period end	$ 1,160,104	$ 1,132,567
Average amount outstanding during period	1,694,804	1,478,114
Maximum amount at any month end during period	$ 2,410,258	$ 2,061,652
Weighted-average interest at period end	2.48%	1.35%
Weighted-average interest rate during period	2.02%	1.00%