Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Financial Assets
Securities available for sale, at fair value	$ 4,873,778	$ 4,783,579
Mortgage loans held for sale, at fair value	128,451	107,734
Mortgage loans held for investment, at fair value option	550	3,143
Derivative instruments	51,086	27,048
Cash and cash equivalents	671,897	690,453	$ 564,092	$ 625,724
Securities held to maturity	198,958	207,446
Financial Liabilities
Derivative instruments	16,756	22,124
Deposits	24,092,062	23,763,431	$ 22,971,192
Short-term borrowings	1,106,131	1,482,882
Long-term debt	1,475,455	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	671,897	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	261,131	315,882
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,873,778	4,783,579
Mortgage loans held for sale, at fair value	128,451	107,734
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	51,086	27,048
Cash and cash equivalents	0	0
Securities held to maturity	200,568	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	16,756	22,124
Deposits	24,086,321	23,752,139
Short-term borrowings	845,000	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	550	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	22,547,830	22,088,236
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,471,292	1,154,062
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,873,778	4,783,579
Mortgage loans held for sale, at fair value	128,451	107,734
Mortgage loans held for investment, at fair value option	550	3,143
Derivative instruments	51,086	27,048
Cash and cash equivalents	671,897	690,453
Securities held to maturity	198,958	207,446
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	22,824,780	22,376,101
Financial Liabilities
Derivative instruments	16,756	22,124
Deposits	24,092,062	23,763,431
Short-term borrowings	1,106,131	1,482,882
Long-term debt	1,475,455	1,166,151
Fair Value
Financial Assets
Securities available for sale, at fair value	4,873,778	4,783,579
Mortgage loans held for sale, at fair value	128,451	107,734
Mortgage loans held for investment, at fair value option	550	3,143
Derivative instruments	51,086	27,048
Cash and cash equivalents	671,897	690,453
Securities held to maturity	200,568	204,277
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	22,547,830	22,088,236
Financial Liabilities
Derivative instruments	16,756	22,124
Deposits	24,086,321	23,752,139
Short-term borrowings	1,106,131	1,482,882
Long-term debt	$ 1,471,292	$ 1,154,062