Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Interest bearing deposits with banks	$ 172,800	$ 171,710
Investment securities held to maturity	1,837,620	1,924,597
Accrued interest receivable	73,990	66,816
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Deposits with stated maturities	3,563,521	3,138,871
Carrying Amount
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	1,842,691	1,925,572
Carrying Amount | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and due from banks	243,310	220,791
Interest bearing deposits with banks	172,800	171,710
Accrued interest receivable	73,990	66,816
Federal Reserve Bank and Federal Home Loan Bank stock	178,668	147,127
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Deposits without stated maturities	14,589,941	14,591,837
Short-term borrowings	748,628	1,080,960
Accrued interest payable	14,161	10,675
Carrying Amount | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	138,676	138,830
Carrying Amount | Significant Other Observable Inputs (Level 2)
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Deposits with stated maturities	3,563,521	3,138,871
Long-term borrowings	2,315,819	1,433,906
Junior subordinated debentures issued to capital trusts	41,774	41,577
Carrying Amount | Significant Other Observable Inputs (Level 2) | U.S. government agency securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	9,859	11,329
Carrying Amount | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	465,878	566,590
Carrying Amount | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	1,131,945	1,112,460
Carrying Amount | Significant Other Observable Inputs (Level 2) | Trust preferred securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	49,824	59,804
Carrying Amount | Significant Other Observable Inputs (Level 2) | Corporate and other debt securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	46,509	36,559
Carrying Amount | Significant Unobservable Inputs (Level 3)
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Net loans	18,210,724	17,121,684
Fair Value
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	1,837,620	1,924,597
Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and due from banks	243,310	220,791
Interest bearing deposits with banks	172,800	171,710
Accrued interest receivable	73,990	66,816
Federal Reserve Bank and Federal Home Loan Bank stock	178,668	147,127
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Deposits without stated maturities	14,589,941	14,591,837
Short-term borrowings	679,316	1,081,751
Accrued interest payable	14,161	10,675
Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	145,257	147,495
Fair Value | Significant Other Observable Inputs (Level 2)
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Deposits with stated maturities	3,465,373	3,160,572
Long-term borrowings	2,453,797	1,523,386
Junior subordinated debentures issued to capital trusts	37,289	45,785
Fair Value | Significant Other Observable Inputs (Level 2) | U.S. government agency securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	9,981	11,464
Fair Value | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	477,479	577,826
Fair Value | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	1,118,044	1,102,802
Fair Value | Significant Other Observable Inputs (Level 2) | Trust preferred securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	40,088	47,290
Fair Value | Significant Other Observable Inputs (Level 2) | Corporate and other debt securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Investment securities held to maturity	46,771	37,720
Fair Value | Significant Unobservable Inputs (Level 3)
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Net loans	$ 17,562,153	$ 16,756,655