Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financial Assets
Securities available for sale, at fair value	$ 3,914,960	$ 3,933,360
Mortgage loans held for sale, at fair value	207,845	213,357
Mortgage loans held for investment, at fair value option	2,779	2,792
Derivative instruments	283,525	104,151
Cash and cash equivalents	945,062	894,723
Securities held to maturity	177,960	182,961
Financial Liabilities
Derivative instruments	23,798	11,799
Deposits	25,526,237	25,219,349	$ 24,092,062
Short-term borrowings	390,747	204,208
Long-term debt	1,288,172	1,343,687
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	945,062	894,723
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	172,747	204,208
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	3,914,960	3,933,360
Mortgage loans held for sale, at fair value	207,845	213,357
Mortgage loans held for investment, at fair value option	0	0
Derivative instruments	283,525	104,151
Cash and cash equivalents	0	0
Securities held to maturity	186,396	189,899
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	0	0
Financial Liabilities
Derivative instruments	23,798	11,799
Deposits	25,547,705	25,229,566
Short-term borrowings	218,000	0
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	2,779	2,792
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	24,209,165	23,732,650
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,261,688	1,296,696
Carrying Amount
Financial Assets
Securities available for sale, at fair value	3,914,960	3,933,360
Mortgage loans held for sale, at fair value	207,845	213,357
Mortgage loans held for investment, at fair value option	2,779	2,792
Derivative instruments	283,525	104,151
Cash and cash equivalents	945,062	894,723
Securities held to maturity	177,960	182,961
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	24,254,947	23,874,911
Financial Liabilities
Derivative instruments	23,798	11,799
Deposits	25,526,237	25,219,349
Short-term borrowings	390,747	204,208
Long-term debt	1,288,172	1,343,687
Fair Value
Financial Assets
Securities available for sale, at fair value	3,914,960	3,933,360
Mortgage loans held for sale, at fair value	207,845	213,357
Mortgage loans held for investment, at fair value option	2,779	2,792
Derivative instruments	283,525	104,151
Cash and cash equivalents	945,062	894,723
Securities held to maturity	186,396	189,899
Loans and leases, carried at amortized cost, net of unearned income and allowance for loan and lease losses	24,209,165	23,732,650
Financial Liabilities
Derivative instruments	23,798	11,799
Deposits	25,547,705	25,229,566
Short-term borrowings	390,747	204,208
Long-term debt	$ 1,261,688	$ 1,296,696