Business Combinations (Schedule of Fair Value of Net Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 861,291	$ 790,972	$ 745,523
MidSouth [Member]
Business Acquisition [Line Items]
Cash and due from banks	28,060
Interest bearing bank deposits	318,914
Federal funds sold	3,475
Securities available for sale	272,406
Loans	785,204
Property and equipment	30,164
Other real estate	435
Identifiable intangible assets	35,000
Other assets	56,860
Total identifiable assets	1,530,518
Deposit liabilities	1,281,078
Short term borrowings	66,996
Long term debt	13,919
Other liabilities	43,883
Total liabilities	1,405,876
Net assets acquired	124,642
Value of stock-based consideration	193,849
Goodwill	$ 69,207