Business Combinations (Schedule of Goodwill) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill, Beginning Balance	$ 790,972	$ 745,523
Initial goodwill recorded - acquisition of trust and asset management business		45,634
Measurement period adjustments - acquisition of trust and asset management business		(185)
Final settlement of cash consideration - acquisition of trust and asset management business	1,112
Goodwill, Ending Balance	861,291	$ 790,972
MidSouth [Member]
Business Acquisition [Line Items]
Initial goodwill recorded - acquisition of trust and asset management business	69,207
Goodwill, Ending Balance	$ 69,207