Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized cost, due in one year, available for sale	$ 600
Amortized cost, due after one year through five years, available for sale	17,981
Amortized cost, due after five years through ten years, available for sale	114,403
Amortized cost, due after ten years, available for sale	854,511
Amortized Cost	987,495	$ 946,751
Estimated fair value, due in one year, available for sale	600
Estimated fair value, due after one year through five years, available for sale	18,690
Estimated fair value, due after five years through ten years, available for sale	113,718
Estimated fair value, due after ten years, available for sale	863,272
Estimated Fair Value	996,280	950,138
Amortized cost, due within one year, held to maturity	1,287
Amortized cost, due after one year through five years, held to maturity	0
Amortized cost, due after five years through ten years, held to maturity	21,156
Amortized cost, due after ten years, held to maturity	314,722
Amortized cost, held to maturity	337,165	375,606
Estimated fair value, due within one year, held to maturity	1,289
Estimated fair value, due after one year through five years, held to maturity	0
Estimated fair value, due after five years through ten years, held to maturity	22,231
Estimated fair value, due after ten years, held to maturity	330,659
Estimated fair value, held to maturity	$ 354,179	$ 381,525