Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest bearing deposits with banks	$ 93,726	$ 172,800
Total investment securities held to maturity	2,016,354	1,837,620
Accrued interest receivable	92,666	73,990
Deposits with stated maturities	5,416,571	3,563,521
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,072,363	1,842,691
Carrying Amount | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	262,653	243,310
Interest bearing deposits with banks	93,726	172,800
Accrued interest receivable	92,666	73,990
Federal Reserve Bank and Federal Home Loan Bank stock	271,905	178,668
Deposits without stated maturities	17,171,701	14,589,941
Short-term borrowings	2,968,431	748,628
Accrued interest payable	17,844	14,161
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	138,557	138,676
Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	5,416,571	3,563,521
Long-term borrowings	1,728,805	2,315,819
Junior subordinated debentures issued to capital trusts	55,283	41,774
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	8,846	9,859
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	596,360	465,878
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	1,259,772	1,131,945
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	37,328	49,824
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	31,500	46,509
Carrying Amount | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	23,966,327	18,210,724
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities held to maturity	2,016,354	1,837,620
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and due from banks	262,653	243,310
Interest bearing deposits with banks	93,726	172,800
Accrued interest receivable	92,666	73,990
Federal Reserve Bank and Federal Home Loan Bank stock	271,905	178,668
Deposits without stated maturities	17,171,701	14,589,941
Short-term borrowings	2,786,568	679,316
Accrued interest payable	17,844	14,161
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	140,269	145,257
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with stated maturities	5,351,921	3,465,373
Long-term borrowings	1,639,788	2,453,797
Junior subordinated debentures issued to capital trusts	47,715	37,289
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	8,541	9,981
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	593,423	477,479
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	1,211,756	1,118,044
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	31,153	40,088
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	31,212	46,771
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net loans	$ 23,104,688	$ 17,562,153