Fair Value Measurements - Valuation Techniques, Significant Unobservable Inputs, Quantitative Information Non-recurring Basis (Detail) - Fair Value, Measurements, Nonrecurring - Fair Value, Inputs, Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FAIR VALUE	$ 12.2	$ 6.7
VALUATION TECHNIQUE	Market Approach
UNOBSERVABLE INPUT	Discount to reflect realizable value
Loans | Lower Limit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RANGE OF DISCOUNTS APPLIED	15.00%
Loans | Upper Limit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RANGE OF DISCOUNTS APPLIED	25.00%
OREO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FAIR VALUE	$ 0.3	$ 0.7
VALUATION TECHNIQUE	Market Approach
UNOBSERVABLE INPUT	Discount to reflect realizable value
OREO | Lower Limit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RANGE OF DISCOUNTS APPLIED	15.00%
OREO | Upper Limit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
RANGE OF DISCOUNTS APPLIED	20.00%