Fair Value of Financial Instruments - Estimated Fair Values and Carrying Amounts of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial Assets
Securities available for sale, at fair value	$ 4,542,486	$ 4,590,062
Equity Securities, FV-NI	9,801	0
Financial Assets	110,348	134,916
Derivative Asset	17,072	31,265
Cash and cash equivalents	564,092	625,724	$ 1,301,118	$ 1,362,126
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	224,241	227,318
Financial Liabilities
Financial Liabilities	22,971,192	21,466,717	$ 17,312,265
Short-term Debt	900,496	991,297
Long-term debt	1,449,302	1,495,835
Derivative instruments	32,154	25,154
Level 1
Financial Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Financial Assets	0	0
Derivative Asset	0	0
Cash and cash equivalents	564,092	625,724
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	0	0
Loans and Leases Receivable, Net Amount	0	0
Financial Liabilities
Financial Liabilities	0	0
Short-term Debt	525,496	516,297
Long-term debt	0	0
Derivative instruments	0	0
Level 2
Financial Assets
Securities available for sale, at fair value	4,542,486	4,590,062
Equity Securities, FV-NI	9,801
Financial Assets	110,348	134,916
Derivative Asset	17,072	31,265
Cash and cash equivalents	0	0
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	219,868	227,964
Loans and Leases Receivable, Net Amount	0	0
Financial Liabilities
Financial Liabilities	22,970,554	21,460,782
Short-term Debt	375,000	475,000
Long-term debt	0	0
Derivative instruments	32,154	25,154
Level 3
Financial Assets
Securities available for sale, at fair value	0	0
Equity Securities, FV-NI	0
Financial Assets	0	0
Derivative Asset	0	0
Cash and cash equivalents	0	0
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	0	0
Loans and Leases Receivable, Net Amount	21,495,639	19,826,857
Financial Liabilities
Financial Liabilities	0	0
Short-term Debt	0	0
Long-term debt	1,429,335	1,476,899
Derivative instruments	0	0
Carrying Amount
Financial Assets
Securities available for sale, at fair value	4,542,486	4,590,062
Equity Securities, FV-NI	9,801
Financial Assets	110,348	134,916
Derivative Asset	17,072	31,265
Cash and cash equivalents	564,092	625,724
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	224,241	227,318
Loans and Leases Receivable, Net Amount	21,561,563	19,937,290
Financial Liabilities
Financial Liabilities	22,971,192	21,466,717
Short-term Debt	900,496	991,297
Long-term debt	1,449,302	1,495,835
Derivative instruments	32,154	25,154
Fair Value
Financial Assets
Securities available for sale, at fair value	4,542,486	4,590,062
Equity Securities, FV-NI	9,801
Financial Assets	110,348	134,916
Derivative Asset	17,072	31,265
Cash and cash equivalents	564,092	625,724
Securities held to maturity (fair values of $219,868 and $227,964, respectively)	219,868	227,964
Loans and Leases Receivable, Net Amount	21,495,639	19,826,857
Financial Liabilities
Financial Liabilities	22,970,554	21,460,782
Short-term Debt	900,496	991,297
Long-term debt	1,429,335	1,476,899
Derivative instruments	$ 32,154	$ 25,154