Goodwill and Other Intangible Assets - Carrying Amounts of Goodwill by Operating Segments (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2017	$ 470,534
Addition to goodwill from acquisition	140,512
Adjustment to previously recorded goodwill	0
Balance at September 30, 2017	611,046
Community Banks
Goodwill [Roll Forward]
Balance at January 1, 2017	467,767
Addition to goodwill from acquisition	140,512
Adjustment to previously recorded goodwill	0
Balance at September 30, 2017	608,279
Insurance
Goodwill [Roll Forward]
Balance at January 1, 2017	2,767
Addition to goodwill from acquisition	0
Adjustment to previously recorded goodwill	0
Balance at September 30, 2017	$ 2,767