Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2021	$ 939,683
Additions to goodwill and other adjustments	0
Balance at September 30, 2021	939,683
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2021	936,916
Additions to goodwill and other adjustments	0
Balance at September 30, 2021	936,916
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2021	2,767
Additions to goodwill and other adjustments	0
Balance at September 30, 2021	$ 2,767