Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest bearing deposits with banks	$ 178,905	$ 177,088
Total investment securities held to maturity	2,184,792	2,034,943
Accrued interest receivable	99,065	95,296
Deposits with stated maturities	7,338,188	7,063,984
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,168,236	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	276,291	251,541
Interest bearing deposits with banks	178,905	177,088
Accrued interest receivable	99,065	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	260,490	232,080
Deposits without stated maturities	17,435,741	17,388,990
Short-term borrowings	2,387,784	2,118,914
Accrued interest payable	36,031	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	138,436	138,517
Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,338,188	7,063,984
Long-term borrowings	1,800,182	1,654,268
Junior subordinated debentures issued to capital trusts	55,544	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	8,049	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	536,778	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	1,415,409	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	37,314	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	25,647,057	24,883,610
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,184,792	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	276,291	251,541
Interest bearing deposits with banks	178,905	177,088
Accrued interest receivable	99,065	95,296
Federal Reserve Bank and Federal Home Loan Bank stock	260,490	232,080
Deposits without stated maturities	17,435,741	17,388,990
Short-term borrowings	2,388,299	2,091,892
Accrued interest payable	36,031	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	144,655	142,049
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	7,338,945	7,005,573
Long-term borrowings	1,840,862	1,751,194
Junior subordinated debentures issued to capital trusts	57,040	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	8,092	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	549,050	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	1,419,066	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	31,372	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	32,557	31,129
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	$ 25,207,831	$ 24,068,755