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Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Securities available for sale, at fair value	$ 3,933,360	$ 4,783,579
Mortgage loans held for sale, at fair value	213,357	107,734	$ 134,916	$ 157,041
Mortgage loans held for investment, at fair value option	2,792	3,143
Derivative instruments	104,151	27,048
Cash and cash equivalents	894,723	690,453
Securities held to maturity	182,961	207,446
Financial Liabilities
Derivative instruments	11,799	22,124
Deposits	25,219,349	23,763,431	21,466,717
Short-term borrowings	204,208	1,482,882	991,297
Long-term debt	1,343,687	1,166,151
Level 1
Financial Assets
Securities available 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	894,723	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	204,208	315,882
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	3,933,360	4,783,579
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	104,151	27,048
Cash and cash equivalents	0	0
Securities held to maturity	189,899	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	11,799	22,124
Deposits	25,229,566	23,752,139
Short-term borrowings	0	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 investment, at fair value option	2,792	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	23,732,650	22,088,236
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,296,696	1,154,062
Carrying Amount
Financial Assets
Securities available for sale, at fair value	3,933,360	4,783,579
Mortgage loans held for investment, at fair value option	2,792	3,143
Derivative instruments	104,151	27,048
Cash and cash equivalents	894,723	690,453
Securities held to maturity	182,961	207,446
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	23,874,911	22,376,101
Financial Liabilities
Derivative instruments	11,799	22,124
Deposits	25,219,349	23,763,431
Short-term borrowings	204,208	1,482,882
Long-term debt	1,343,687	1,166,151
Fair Value
Financial Assets
Securities available for sale, at fair value	3,933,360	4,783,579
Mortgage loans held for investment, at fair value option	2,792	3,143
Derivative instruments	104,151	27,048
Cash and cash equivalents	894,723	690,453
Securities held to maturity	189,899	204,277
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	23,732,650	22,088,236
Financial Liabilities
Derivative instruments	11,799	22,124
Deposits	25,229,566	23,752,139
Short-term borrowings	204,208	1,482,882
Long-term debt	1,296,696	1,154,062
Mortgage loans held for sale
Financial Assets
Mortgage loans held for sale, at fair value	213,357	107,734	$ 134,916
Mortgage loans held for sale \| Level 1
Financial Assets
Mortgage loans held for sale, at fair value	0	0
Mortgage loans held for sale \| Level 2
Financial Assets
Mortgage loans held for sale, at fair value	213,357	107,734
Mortgage loans held for sale \| Level 3
Financial Assets
Mortgage loans held for sale, at fair value	0	0
Mortgage loans held for sale \| Carrying Amount
Financial Assets
Mortgage loans held for sale, at fair value	213,357	107,734
Mortgage loans held for sale \| Fair Value
Financial Assets
Mortgage loans held for sale, at fair value	$ 213,357	$ 107,734