Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 1
Due after one year through five years, Amortized Cost	9,392
Due after five years through ten years, Amortized Cost	14,548
Due after ten years, Amortized Cost	535,162
Totals, Amortized Cost	559,103
Due in one year, Estimated Fair Value	1
Due after one year through five years, Estimated Fair Value	9,650
Due after five years through ten years, Estimated Fair Value	15,132
Due after ten years, Estimated Fair Value	525,450
Totals, Estimated Fair Value	550,233
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,177
Due after five years through ten years, Amortized Cost, Held to Maturity	4,776
Due after ten years, Amortized Cost, Held to Maturity	596,583
Totals, Amortized Cost, Held to Maturity	602,536	$ 726,530
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	1,185
Due after five years through ten years, Estimated Fair Value, Held to Maturity	4,762
Due after ten years, Estimated Fair Value, Held to Maturity	597,256
Estimated Fair Value, Held to Maturity	$ 603,203	$ 732,209