Fair Value Measurements - Quantitative Information Related to Significant Unobservable Inputs (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Privately Issued [Member] | Two Independent Pricing Quotes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Recurring fair value measurements for certain Level 3 Assets and Liabilities	$ 28	$ 44
Other Assets and Other Liabilities [Member] | Discounted Cash Flows [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Recurring fair value measurements for certain Level 3 Assets and Liabilities	$ 8,303	$ 7,325
Other Assets and Other Liabilities [Member] | Discounted Cash Flows [Member] | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Commitment expirations	0.00%	0.00%
Other Assets and Other Liabilities [Member] | Discounted Cash Flows [Member] | Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Commitment expirations	78.00%	77.00%
Other Assets and Other Liabilities [Member] | Discounted Cash Flows [Member] | Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Commitment expirations	22.00%	30.00%