Fair Value Measurements - Book Value and Estimated Fair Value of Bank's Significant Financial Instruments Not Recorded at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Assets:
Securities held-to-maturity, net estimated fair value		$ 761,660
Restricted equity investments, at cost	50,418	19,724
Financial Liabilities:
Time deposits	929,083	607,104
Securities sold under agreements to repurchase with retail customers	82,463	79,668
Book Value
Financial Assets:
Cash and due from banks	119,364	109,613
Securities held-to-maturity, net estimated fair value	982,857	764,062
Restricted equity investments, at cost	50,418	19,724
Loans receivable, net and mortgage loans held for sale	5,413,947	3,966,014
Financial Liabilities:
Deposits other than time deposits	4,978,253	3,735,694
Time deposits	929,083	607,104
Securities sold under agreements to repurchase with retail customers	82,463	79,668
Federal Home Loan Bank advances and other borrowings	441,005	345,210
Level 1 Inputs
Financial Assets:
Cash and due from banks	119,364	109,613
Financial Liabilities:
Securities sold under agreements to repurchase with retail customers	82,463	79,668
Level 2 Inputs
Financial Assets:
Securities held-to-maturity, net estimated fair value	959,574	751,182
Financial Liabilities:
Deposits other than time deposits	4,978,253	3,735,694
Time deposits	921,845	599,677
Federal Home Loan Bank advances and other borrowings	432,368	341,820
Level 3 Inputs
Financial Assets:
Securities held-to-maturity, net estimated fair value	11,825	10,478
Restricted equity investments, fair value	50,418	19,724
Loans receivable, net and mortgage loans held for sale	$ 5,326,600	$ 3,962,689