Goodwill and Other Intangible Assets - Summary of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, Beginning Balance	$ 172,559	$ 112,657
Goodwill, Acquired During Period	0	59,902
Goodwill, Ending Balance	172,559	172,559
Banking [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	138,476	78,574
Goodwill, Acquired During Period	0	59,902
Goodwill, Ending Balance	138,476	138,476
Wealth Management [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	15,434	15,434
Goodwill, Acquired During Period	0	0
Goodwill, Ending Balance	15,434	15,434
Insurance [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	18,649	18,649
Goodwill, Acquired During Period	0	0
Goodwill, Ending Balance	$ 18,649	$ 18,649