Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2019	$ 932,928
Measurement period adjustment to goodwill from Brand acquisition	6,755
Balance at September 30, 2019	939,683
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2019	930,161
Measurement period adjustment to goodwill from Brand acquisition	6,755
Balance at September 30, 2019	936,916
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2019	2,767
Measurement period adjustment to goodwill from Brand acquisition	0
Balance at September 30, 2019	$ 2,767