Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2020	$ 939,683
Adjustment to previously recorded goodwill	0
Balance at March 31, 2020	939,683
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2020	936,916
Adjustment to previously recorded goodwill	0
Balance at March 31, 2020	936,916
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2020	2,767
Adjustment to previously recorded goodwill	0
Balance at March 31, 2020	$ 2,767