Fair Value Measurements - Significant Unobservable Inputs in Fair Value Measurement Level 3 Assets and Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, fair value	$ 14,072	$ 14,906
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, fair value	8,584,396	7,877,094
Fair Value, Measurements, Nonrecurring | Level 3 | Impaired loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, fair value	5,533	6,153
Fair Value, Measurements, Nonrecurring | Level 3 | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, fair value	$ 14,072	$ 14,906
Range (Weighted Average)	10.00%
Fair Value, Measurements, Nonrecurring | Level 3 | Minimum | Impaired loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (Weighted Average)	0.00%
Fair Value, Measurements, Nonrecurring | Level 3 | Maximum | Impaired loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (Weighted Average)	100.00%
Fair Value, Measurements, Nonrecurring | Level 3 | Weighted Average | Impaired loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (Weighted Average)	8.50%