Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest bearing deposits with banks	$ 654,591	$ 178,423
Equity securities	29,026	41,410
Total investment securities held to maturity	2,224,971	2,358,720
Accrued interest receivable	136,058	105,637
Deposits with stated maturities	7,537,581	9,717,945
NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	7,400
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,170,476	2,336,095
Carrying Amount | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	276,120	256,264
Interest bearing deposits with banks	654,591	178,423
Accrued interest receivable	136,058	105,637
Deposits without stated maturities	23,650,401	19,467,892
Short-term borrowings	1,430,726	1,093,280
Accrued interest payable	21,756	33,066
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	138,224	138,352
Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Federal Reserve Bank and Federal Home Loan Bank stock	287,628	214,421
Deposits with stated maturities	7,537,581	9,717,945
Long-term borrowings	2,852,569	2,122,426
Junior subordinated debentures issued to capital trusts	55,978	55,718
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	6,302	7,345
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	464,051	500,705
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	1,491,808	1,620,119
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	37,341	37,324
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	32,750	32,250
Carrying Amount | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2,499	0
Net loans	32,090,554	29,537,449
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,224,971	2,358,720
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	276,120	256,264
Interest bearing deposits with banks	654,591	178,423
Accrued interest receivable	136,058	105,637
Deposits without stated maturities	23,650,401	19,467,892
Short-term borrowings	1,544,596	1,081,879
Accrued interest payable	21,756	33,066
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	146,429	144,113
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Federal Reserve Bank and Federal Home Loan Bank stock	287,628	214,421
Deposits with stated maturities	7,466,794	9,747,867
Long-term borrowings	3,115,757	2,181,401
Junior subordinated debentures issued to capital trusts	44,771	53,889
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	6,634	7,362
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	479,824	513,607
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	1,528,703	1,629,572
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	30,000	31,382
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	33,381	32,684
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2,499	0
Net loans	$ 31,883,095	$ 28,964,396