Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 133,340	$ 190,387	$ 149,957
FEDERAL FUNDS SOLD	2,790	8,760	4,785
INTEREST-BEARING DEPOSITS IN BANKS	4,268	54,324	83,994
INTEREST-BEARING TIME DEPOSITS IN BANKS	4,491	17,002	19,234
Available-for-sale Securities	2,737,353	2,415,856	2,253,762
Held-to-maturity securities	286	441	554
Loans	3,248,002	2,901,167	2,803,308
SHORT-TERM BORROWINGS	500,903	367,110	341,909
Carrying Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	133,340	190,387	149,957
FEDERAL FUNDS SOLD	2,790	8,760	4,785
INTEREST-BEARING DEPOSITS IN BANKS	4,268	54,324	83,994
Deposits with no stated maturities	4,444,364	4,101,263	3,800,143
Carrying Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	4,491	17,002	19,234
Held-to-maturity securities	286	441	554
Accrued interest receivable	26,888	28,998	23,420
Deposits with stated maturities	652,919	648,992	664,221
SHORT-TERM BORROWINGS	500,903	367,110	341,909
Accrued interest payable	261	237	275
Carrying Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,737,353	2,415,856	2,253,762
Carrying Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,248,002	2,901,167	2,803,308
Estimated Fair Value [Member] | Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	133,340	190,387	149,957
FEDERAL FUNDS SOLD	2,790	8,760	4,785
INTEREST-BEARING DEPOSITS IN BANKS	4,268	54,324	83,994
Deposits with no stated maturities	4,444,364	4,101,263	3,800,143
Estimated Fair Value [Member] | Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	4,498	17,032	19,285
Held-to-maturity securities	291	447	560
Accrued interest receivable	26,888	28,998	23,420
Deposits with stated maturities	654,705	650,893	666,123
SHORT-TERM BORROWINGS	500,903	367,110	341,909
Accrued interest payable	261	237	275
Estimated Fair Value [Member] | Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,737,353	2,415,856	2,253,762
Estimated Fair Value [Member] | Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,249,558	$ 2,906,399	$ 2,815,509