Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 02, 2016
Goodwill And Other Intangibles [Line Items]
Goodwill	$ 186,483,000	$ 164,500,000
Impairment charges on goodwill	0	0	$ 0
Total amortization	19,442,000	18,895,000
Amortization of other intangible assets	547,000	605,000	$ 631,000
FirstMerit Bank, NA, Branch Acquisition [Member]
Goodwill And Other Intangibles [Line Items]
Goodwill				$ 22,047,000
Goodwill, Acquired During Period	$ 22,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles				$ 11,330,000
Customer deposit intangibles
Goodwill And Other Intangibles [Line Items]
Core deposits remaining amortization period	9 years 11 months
Core deposits weighted average amortization period	10 years
Customer list intangible
Goodwill And Other Intangibles [Line Items]
Core deposits weighted average amortization period	12 years 10 months
Total amortization	$ 444,000	$ 276,000