Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest bearing deposits with banks	$ 184,347	$ 177,088
Total investment securities held to maturity	2,066,970	2,034,943
Accrued interest receivable	100,722	95,296
Deposits with stated maturities	7,108,318	7,063,984
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,074,399	2,068,246
Carrying Amount | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	282,250	251,541
Interest bearing deposits with banks	184,347	177,088
Accrued interest receivable	100,722	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	222,630	232,080
Deposits without stated maturities	17,799,178	17,388,990
Short-term borrowings	2,062,576	2,118,914
Accrued interest payable	33,933	25,762
Carrying Amount | Level 1 | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	138,478	138,517
Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,108,318	7,063,984
Long-term borrowings	1,499,727	1,654,268
Junior subordinated debentures issued to capital trusts	55,457	55,370
Carrying Amount | Level 2 | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	8,236	8,721
Carrying Amount | Level 2 | Obligations of states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	551,501	585,656
Carrying Amount | Level 2 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	1,306,625	1,266,770
Carrying Amount | Level 2 | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	37,309	37,332
Carrying Amount | Level 2 | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	32,250	31,250
Carrying Amount | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	25,268,737	24,883,610
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,066,970	2,034,943
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	282,250	251,541
Interest bearing deposits with banks	184,347	177,088
Accrued interest receivable	100,722	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	222,630	232,080
Deposits without stated maturities	17,799,178	17,388,990
Short-term borrowings	2,006,439	2,091,892
Accrued interest payable	33,933	25,762
Fair Value | Level 1 | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	142,899	142,049
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,127,215	7,005,573
Long-term borrowings	1,615,030	1,751,194
Junior subordinated debentures issued to capital trusts	55,474	55,692
Fair Value | Level 2 | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	8,069	8,641
Fair Value | Level 2 | Obligations of states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	559,838	586,033
Fair Value | Level 2 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	1,292,364	1,235,605
Fair Value | Level 2 | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	31,601	31,486
Fair Value | Level 2 | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	32,199	31,129
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	$ 24,766,120	$ 24,068,755