Disclosures about Fair Value of Assets and Liabilities - Qualitative Information about Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements, Other than Goodwill (Detail) - Significant Unobservable Inputs (Level 3) [Member] $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) ConsecutivePayment	Dec. 31, 2018 USD ($) ConsecutivePayment
Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value | $	$ 7,847	$ 5,661
Valuation Technique	Collateral based measurement	Collateral based measurement
Impaired loans	Discount to reflect current market conditions and ultimate collectability	Discount to reflect current market conditions and ultimate collectability
Mortgage Servicing Rights [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value | $	$ 13,652	$ 12,349
Valuation Technique	Discounted cash flows	Discounted cash flows
Impaired loans	Discount rate, Constant prepayment rate, Probability of default	Discount rate, Constant prepayment rate, Probability of default
Minimum [Member] | Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount to reflect current market conditions and ultimate collectability	0.00%	0.00%
Minimum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	9.7	10.2
Minimum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	9.6	9.1
Minimum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Default Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	0.0	0.1
Maximum [Member] | Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount to reflect current market conditions and ultimate collectability	100.00%	100.00%
Maximum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	10.0	11.0
Maximum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	19.3	21.9
Maximum [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Default Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	1.7	2.8
Weighted Average [Member] | Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount to reflect current market conditions and ultimate collectability	9.10%	15.50%
Weighted Average [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	9.8	10.3
Weighted Average [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	11.0	9.3
Weighted Average [Member] | Mortgage Servicing Rights [Member] | Measurement Input, Default Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable inputs	0.7	0.6