Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Financial Assets
Securities available for sale, at fair value	$ 4,650,915	$ 4,590,062
CRA and Community Development Investment Funds	1,878	0
Mortgage loans held for sale	78,843	134,916
Derivative Asset	12,913	31,265
Cash and cash equivalents	727,388	625,724	$ 469,360	$ 1,362,126
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	221,030	227,318
Financial Liabilities
Derivative instruments	38,607	25,154
Deposits	23,430,458	21,466,717	$ 16,853,116
Short-term Debt	1,054,213	991,297
Long-term debt	1,438,614	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 Asset	0	0
Cash and cash equivalents	727,388	625,724
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	0	0
Loans and Leases Receivable, Net Amount	0	0
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term Debt	459,213	516,297
Long-term debt	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,650,915	4,590,062
CRA and Community Development Investment Funds	1,878
Mortgage loans held for sale	78,843	134,916
Derivative Asset	12,913	31,265
Cash and cash equivalents	0	0
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	216,309	227,964
Loans and Leases Receivable, Net Amount	0	0
Financial Liabilities
Derivative instruments	38,607	25,154
Deposits	23,425,170	21,460,782
Short-term Debt	595,000	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 Asset	0	0
Cash and cash equivalents	0	0
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	0	0
Loans and Leases Receivable, Net Amount	21,825,925	19,826,857
Financial Liabilities
Derivative instruments	0	0
Deposits	0	0
Short-term Debt	0	0
Long-term debt	1,423,382	1,476,899
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,650,915	4,590,062
CRA and Community Development Investment Funds	1,878
Mortgage loans held for sale	78,843	134,916
Derivative Asset	12,913	31,265
Cash and cash equivalents	727,388	625,724
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	221,030	227,318
Loans and Leases Receivable, Net Amount	21,939,207	19,937,290
Financial Liabilities
Derivative instruments	38,607	25,154
Deposits	23,430,458	21,466,717
Short-term Debt	1,054,213	991,297
Long-term debt	1,438,614	1,495,835
Fair Value
Financial Assets
Securities available for sale, at fair value	4,650,915	4,590,062
CRA and Community Development Investment Funds	1,878
Mortgage loans held for sale	78,843	134,916
Derivative Asset	12,913	31,265
Cash and cash equivalents	727,388	625,724
Securities held to maturity (fair values of $216,309 and $227,964, respectively)	216,309	227,964
Loans and Leases Receivable, Net Amount	21,825,925	19,826,857
Financial Liabilities
Derivative instruments	38,607	25,154
Deposits	23,425,170	21,460,782
Short-term Debt	1,054,213	991,297
Long-term debt	$ 1,423,382	$ 1,476,899