Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 139,995	$ 179,140	$ 142,233
FEDERAL FUNDS SOLD	2,660	3,810	5,460
INTEREST-BEARING DEPOSITS IN BANKS	22,993	89,936	18,275
INTEREST-BEARING TIME DEPOSITS IN BANKS	2,427	3,495	9,170
Available-for-sale Securities	2,763,039	2,733,899	2,689,330
Held-to-maturity securities	146	278	310
Loans	3,255,135	3,308,716	2,900,879
SHORT-TERM BORROWINGS	525,340	615,675	401,898
Carrying Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	139,995	179,140	142,233
FEDERAL FUNDS SOLD	2,660	3,810	5,460
INTEREST-BEARING DEPOSITS IN BANKS	22,993	89,936	18,275
Deposits with no stated maturities	4,473,909	4,569,317	4,213,899
Carrying Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	2,427	3,495	9,170
Held-to-maturity securities	146	278	310
Accrued interest receivable	26,166	34,697	25,238
Deposits with stated maturities	589,898	620,852	623,108
SHORT-TERM BORROWINGS	525,340	615,675	401,898
Accrued interest payable	254	240	235
Carrying Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,763,039	2,733,899	2,689,330
Carrying Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,255,135	3,308,716	2,900,879
Estimated Fair Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	139,995	179,140	142,233
FEDERAL FUNDS SOLD	2,660	3,810	5,460
INTEREST-BEARING DEPOSITS IN BANKS	22,993	89,936	18,275
Deposits with no stated maturities	4,473,909	4,569,317	4,213,899
Estimated Fair Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	2,430	3,500	9,185
Held-to-maturity securities	150	283	317
Accrued interest receivable	26,166	34,697	25,238
Deposits with stated maturities	591,675	622,572	624,926
SHORT-TERM BORROWINGS	525,340	615,675	401,898
Accrued interest payable	254	240	235
Estimated Fair Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,763,039	2,733,899	2,689,330
Estimated Fair Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,260,544	$ 3,316,243	$ 2,907,114