Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2019	$ 932,928
Measurement period adjustment to goodwill from previous acquisition	43
Balance at June 30, 2019	932,971
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2019	930,161
Measurement period adjustment to goodwill from previous acquisition	43
Balance at June 30, 2019	930,204
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2019	2,767
Measurement period adjustment to goodwill from previous acquisition	0
Balance at June 30, 2019	$ 2,767