Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest bearing deposits with banks	$ 185,841	$ 177,088
Total investment securities held to maturity	2,121,203	2,034,943
Accrued interest receivable	97,282	95,296
Deposits with stated maturities	7,872,172	7,063,984
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,093,757	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	312,396	251,541
Interest bearing deposits with banks	185,841	177,088
Accrued interest receivable	97,282	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	235,965	232,080
Deposits without stated maturities	17,673,950	17,388,990
Short-term borrowings	1,825,417	2,118,914
Accrued interest payable	30,819	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	138,393	138,517
Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,872,172	7,063,984
Long-term borrowings	2,250,633	1,654,268
Junior subordinated debentures issued to capital trusts	55,631	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	7,462	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	517,430	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	1,360,903	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	37,319	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	26,405,306	24,883,610
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,121,203	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	312,396	251,541
Interest bearing deposits with banks	185,841	177,088
Accrued interest receivable	97,282	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	235,965	232,080
Deposits without stated maturities	17,673,950	17,388,990
Short-term borrowings	1,825,486	2,091,892
Accrued interest payable	30,819	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	144,872	142,049
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,891,568	7,005,573
Long-term borrowings	2,467,403	1,751,194
Junior subordinated debentures issued to capital trusts	55,670	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	7,553	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	530,482	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	1,374,825	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	30,897	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	32,574	31,129
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	$ 25,889,653	$ 24,068,755