Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial Assets
Securities available for sale, at fair value	$ 4,455,308	$ 4,783,579
Mortgage loans held for sale, at fair value	187,987	107,734
Mortgage loans held for investment, at fair value option	1,945	3,143
Derivative instruments	107,747	27,048
Cash and cash equivalents	789,315	690,453	$ 727,388	$ 625,724
Securities held to maturity	192,917	207,446
Financial Liabilities
Derivative instruments	12,599	22,124
Deposits	24,295,331	23,763,431	$ 23,430,458
Short-term borrowings	997,507	1,482,882
Long-term debt	1,374,759	1,166,151
Level 1
Financial Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for sale, at fair value	0	0
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	0	0
Cash and cash equivalents	789,315	690,453
Securities held to maturity	0	0
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	0	0
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	184,507	315,882
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,455,308	4,783,579
Mortgage loans held for sale, at fair value	187,987	107,734
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	107,747	27,048
Cash and cash equivalents	0	0
Securities held to maturity	198,012	204,277
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	0	0
Financial Liabilities
Derivative instruments	12,599	22,124
Deposits	24,295,695	23,752,139
Short-term borrowings	813,000	1,167,000
Long-term debt	0	0
Level 3
Financial Assets
Securities available for sale, at fair value	0	0
Mortgage loans held for sale, at fair value	0	0
Mortgage loans held for investment, at fair value option	1,945	3,143
Derivative instruments	0	0
Cash and cash equivalents	0	0
Securities held to maturity	0	0
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	22,939,128	22,088,236
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,381,572	1,154,062
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,455,308	4,783,579
Mortgage loans held for sale, at fair value	187,987	107,734
Mortgage loans held for investment, at fair value option	1,945	3,143
Derivative instruments	107,747	27,048
Cash and cash equivalents	789,315	690,453
Securities held to maturity	192,917	207,446
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	23,206,980	22,376,101
Financial Liabilities
Derivative instruments	12,599	22,124
Deposits	24,295,331	23,763,431
Short-term borrowings	997,507	1,482,882
Long-term debt	1,374,759	1,166,151
Fair Value
Financial Assets
Securities available for sale, at fair value	4,455,308	4,783,579
Mortgage loans held for sale, at fair value	187,987	107,734
Mortgage loans held for investment, at fair value option	1,945	3,143
Derivative instruments	107,747	27,048
Cash and cash equivalents	789,315	690,453
Securities held to maturity	198,012	204,277
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	22,939,128	22,088,236
Financial Liabilities
Derivative instruments	12,599	22,124
Deposits	24,295,695	23,752,139
Short-term borrowings	997,507	1,482,882
Long-term debt	$ 1,381,572	$ 1,154,062