Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Financial Assets
Securities available for sale, at fair value	$ 4,238,082	$ 4,783,579
Mortgage loans held for sale, at fair value	255,276	107,734
Mortgage loans held for investment, at fair value option	4,138	3,143
Derivative instruments	136,235	27,048
Cash and cash equivalents	930,933	690,453
Securities held to maturity	185,007	207,446
Financial Liabilities
Derivative instruments	10,851	22,124
Deposits	24,977,285	23,763,431	$ 23,193,446
Short-term borrowings	498,049	1,482,882
Long-term debt	1,394,202	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	930,933	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	223,049	315,882
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,238,082	4,783,579
Mortgage loans held for sale, at fair value	255,276	107,734
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	136,235	27,048
Cash and cash equivalents	0	0
Securities held to maturity	192,586	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	10,851	22,124
Deposits	24,985,510	23,752,139
Short-term borrowings	275,000	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	4,138	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	23,290,286	22,088,236
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,391,857	1,154,062
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,238,082	4,783,579
Mortgage loans held for sale, at fair value	255,276	107,734
Mortgage loans held for investment, at fair value option	4,138	3,143
Derivative instruments	136,235	27,048
Cash and cash equivalents	930,933	690,453
Securities held to maturity	185,007	207,446
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	23,526,164	22,376,101
Financial Liabilities
Derivative instruments	10,851	22,124
Deposits	24,977,285	23,763,431
Short-term borrowings	498,049	1,482,882
Long-term debt	1,394,202	1,166,151
Fair Value
Financial Assets
Securities available for sale, at fair value	4,238,082	4,783,579
Mortgage loans held for sale, at fair value	255,276	107,734
Mortgage loans held for investment, at fair value option	4,138	3,143
Derivative instruments	136,235	27,048
Cash and cash equivalents	930,933	690,453
Securities held to maturity	192,586	204,277
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	23,290,286	22,088,236
Financial Liabilities
Derivative instruments	10,851	22,124
Deposits	24,985,510	23,752,139
Short-term borrowings	498,049	1,482,882
Long-term debt	$ 1,391,857	$ 1,154,062