Fair Value Measurement - Quantitative Information About Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Loans held for sale, at fair value	$ 95,235	$ 83,541
Construction [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Loans held for sale, at fair value	95,235	83,541
Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Loans held for sale, at fair value	95,235	83,541
Fair Value, Measurements, Recurring [Member] | Comparable Sales [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Loans held for sale, at fair value	$ 80,927
Fair Value, Measurements, Recurring [Member] | Comparable Sales [Member] | Construction [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Loans held for sale, at fair value		$ 65,016
Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Comparable Sales [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair Value Inputs, Time discount	0.00%
Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Comparable Sales [Member] | Construction [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair Value Inputs, Time discount		0.00%
Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Comparable Sales [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair Value Inputs, Time discount	2.00%
Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Comparable Sales [Member] | Construction [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair Value Inputs, Time discount		1.96%