Significant Unobservable Inputs used in Fair Value Measurement (Detail) - Mortgage servicing rights $ in Thousands		6 Months Ended
		Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value		$ 2,505	$ 2,729	$ 220,586	$ 228,099
Valuation Technique, Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Valuation Technique		Discounted Cash Flow
Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant Unobservable Input Value	[1]	0.1066
Measurement Input, Discount Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Significant Unobservable Input Value		0.1200

[1] Represents annualized loan repayment rate assumptions.