Fair Value Measurements - Book Value and Estimated Fair Value of Bank's Significant Financial Instruments Not Recorded at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial Assets:
Securities held-to-maturity, net estimated fair value	$ 864,173	$ 761,660
Restricted equity investments, at cost	57,143	19,724
Financial Liabilities:
Time deposits	853,111	607,104
Securities sold under agreements to repurchase with retail customers	61,044	79,668
Book Value
Financial Assets:
Cash and due from banks	148,362	109,613
Securities held-to-maturity, net estimated fair value	883,540	764,062
Restricted equity investments, at cost	57,143	19,724
Loans receivable, net and mortgage loans held for sale	5,544,691	3,966,014
Financial Liabilities:
Deposits other than time deposits	5,001,139	3,735,694
Time deposits	853,111	607,104
Federal Home Loan Bank advances and other borrowings	556,279	345,210
Securities sold under agreements to repurchase with retail customers	61,044	79,668
Level 1 Inputs
Financial Assets:
Cash and due from banks	148,362	109,613
Financial Liabilities:
Securities sold under agreements to repurchase with retail customers	61,044	79,668
Level 2 Inputs
Financial Assets:
Securities held-to-maturity, net estimated fair value	852,348	751,182
Financial Liabilities:
Deposits other than time deposits	5,001,139	3,735,694
Time deposits	836,692	599,677
Federal Home Loan Bank advances and other borrowings	559,107	341,820
Level 3 Inputs
Financial Assets:
Securities held-to-maturity, net estimated fair value	11,825	10,478
Restricted equity investments, fair value	57,143	19,724
Loans receivable, net and mortgage loans held for sale	$ 5,464,517	$ 3,962,689