Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Securities available for sale, at fair value	$ 4,634,124	$ 4,590,062
CRA and Community Development Investment Funds	1,866	0
Mortgage loans held for sale	42,976	134,916
Derivative instruments	12,570	31,265
Cash and cash equivalents	475,935	625,724	$ 881,216	$ 1,362,126
Securities held to maturity	213,561	227,318
Financial Liabilities
Derivative instruments	42,764	25,154
Deposits	23,193,446	21,466,717	$ 21,334,271
Short-term borrowings	1,242,719	991,297
Long-term debt	1,466,810	1,495,835
Level 1
Financial Assets
Securities available for sale, at fair value	0	0
CRA and Community Development Investment Funds	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Cash and cash equivalents	475,935	625,724
Securities held to maturity	0	0
Loans and leases, 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	452,719	516,297
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,634,124	4,590,062
CRA and Community Development Investment Funds	1,866
Mortgage loans held for sale	42,976	134,916
Derivative instruments	12,570	31,265
Cash and cash equivalents	0	0
Securities held to maturity	207,309	227,964
Loans and leases, net of unearned income and allowance for loan and lease losses	0	0
Financial Liabilities
Derivative instruments	42,764	25,154
Deposits	23,181,429	21,460,782
Short-term borrowings	790,000	475,000
Long-term debt	0	0
Level 3
Financial Assets
Securities available for sale, at fair value	0	0
CRA and Community Development Investment Funds	0
Mortgage loans held for sale	0	0
Derivative instruments	0	0
Cash and cash equivalents	0	0
Securities held to maturity	0	0
Loans and leases, net of unearned income and allowance for loan and lease losses	22,014,886	19,826,857
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	1,449,018	1,476,899
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,634,124	4,590,062
CRA and Community Development Investment Funds	1,866
Mortgage loans held for sale	42,976	134,916
Derivative instruments	12,570	31,265
Cash and cash equivalents	475,935	625,724
Securities held to maturity	213,561	227,318
Loans and leases, net of unearned income and allowance for loan and lease losses	22,206,956	19,937,290
Financial Liabilities
Derivative instruments	42,764	25,154
Deposits	23,193,446	21,466,717
Short-term borrowings	1,242,719	991,297
Long-term debt	1,466,810	1,495,835
Fair Value
Financial Assets
Securities available for sale, at fair value	4,634,124	4,590,062
CRA and Community Development Investment Funds	1,866
Mortgage loans held for sale	42,976	134,916
Derivative instruments	12,570	31,265
Cash and cash equivalents	475,935	625,724
Securities held to maturity	207,309	227,964
Loans and leases, net of unearned income and allowance for loan and lease losses	22,014,886	19,826,857
Financial Liabilities
Derivative instruments	42,764	25,154
Deposits	23,181,429	21,460,782
Short-term borrowings	1,242,719	991,297
Long-term debt	$ 1,449,018	$ 1,476,899

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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