Investment Securities - Estimated Fair Value of Investment Securities Available for Sale and Held to Maturity by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost
Available-for-sale, Amortized Cost	$ 657,631	$ 640,455
Estimated Fair Value
Available-for-sale, Estimated Fair Value	$ 663,925	637,329
Yield
Available-for-sale, Total, Yield	2.14%
Amortized Cost
Held-to-maturity, Amortized Cost	$ 460,483	472,505
Estimated Fair Value
Held-to-maturity, Estimated Fair Value	$ 467,372	475,134
Yield
Held-to-maturity, Total, Yield	2.83%
Debt Securities
Amortized Cost
Available-for-sale, Amortized Cost, Due in one year or less	$ 0
Available-for-sale, Amortized Cost, Due after one year through five years	0
Available-for-sale, Amortized Cost, Due after five years through ten years	0
Available-for-sale, Amortized Cost, Due after ten years	46,198
Estimated Fair Value
Available-for-sale, Estimated Fair Value, Due in one year or less	0
Available-for-sale, Estimated Fair Value, Due after one year through five years	0
Available-for-sale, Estimated Fair Value, Due after five years through ten years	0
Available-for-sale, Estimated Fair Value, Due after ten years	$ 43,061
Yield
Available-for-sale, Due in one year or less, Yield	0.00%
Available-for-sale, Due after one year through five years, Yield	0.00%
Available-for-sale, Due after five years through ten years, Yield	0.00%
Available-for-sale, Due after ten years, Yield	2.95%
Mortgage-backed securities—residential issued by government sponsored entities
Amortized Cost
Available-for-sale, Amortized Cost, Mortgage-backed securities—residential issued by government sponsored entities	$ 611,433
Available-for-sale, Amortized Cost	611,433	614,176
Estimated Fair Value
Available-for-sale, Estimated Fair Value, Mortgage-backed securities—residential issued by government sponsored entities	620,864
Available-for-sale, Estimated Fair Value	$ 620,864	611,137
Yield
Available-for-sale, Mortgage-backed securities—residential issued by government sponsored entities, Yield	2.09%
Debt Securities
Amortized Cost
Held-to-maturity, Amortized Cost, Due in one year or less	$ 0
Held-to-maturity, Amortized Cost, Due after one year through five years	19,311
Held-to-maturity, Amortized Cost, Due after five years through ten years	65,238
Held-to-maturity, Amortized Cost, Due after ten years	13,017
Estimated Fair Value
Held-to-maturity, Estimated Fair Value, Due in one year or less	0
Held-to-maturity, Estimated Fair Value, Due after one year through five years	18,330
Held-to-maturity, Estimated Fair Value, Due after five years through ten years	64,052
Held-to-maturity, Estimated Fair Value, Due after ten years	$ 13,497
Yield
Held-to-maturity, Due in one year or less, Yield	0.00%
Held-to-maturity, Due after one year through five years, Yield	4.37%
Held-to-maturity, Due after five years through ten years, Yield	4.89%
Held-to-maturity, Due after ten years, Yield	2.87%
Mortgage-backed securities—residential issued by government sponsored entities
Amortized Cost
Held-to-maturity, Amortized Cost, Mortgage-backed securities—residential issued by government sponsored entities	$ 362,917
Held-to-maturity, Amortized Cost	362,917	378,264
Estimated Fair Value
Held-to-maturity, Estimated Fair Value, Mortgage-backed securities—residential issued by government sponsored entities	371,493
Held-to-maturity, Estimated Fair Value	$ 371,493	$ 380,559
Yield
Held-to-maturity, Mortgage-backed securities—residential issued by government sponsored entities, Yield	2.40%