Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2018	$ 611,046
Addition to goodwill from acquisition	316,215
Adjustment to previously recorded goodwill	0
Balance at September 30, 2018	927,261
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2018	608,279
Addition to goodwill from acquisition	316,215
Adjustment to previously recorded goodwill	0
Balance at September 30, 2018	924,494
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2018	2,767
Addition to goodwill from acquisition	0
Adjustment to previously recorded goodwill	0
Balance at September 30, 2018	$ 2,767