Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 14,229,439	$ 12,548,280
Loans and Leases Receivable, Net Amount	24,313,650	22,851,292
Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances	700,000	700,000
Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	395,000	330,000
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	7,140	3,730
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,386	347,303
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	348,345	348,189
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,619,384	2,923,075
Held-to-maturity securities	14,548,856	12,663,455
Non-marketable securities not measured at net asset value	123,811	120,019
Non-marketable securities measured at net asset value	134,942	228,399
FHLB and Federal Reserve Bank stock	60,514	60,020
Non-maturity deposits (1)	45,895,018	44,206,929
Time deposits	41,514	47,146
Commitments to extend credit	0	0
Carrying Amount | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		700,000
Carrying Amount | Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings		330,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	1,102,140	3,730
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,386	347,303
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	348,345	348,189
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,619,384	2,923,075
Held-to-maturity securities	14,229,439	12,548,280
Non-marketable securities not measured at net asset value	123,811	126,345
Non-marketable securities measured at net asset value	134,942	331,496
FHLB and Federal Reserve Bank stock	60,514	60,020
Non-maturity deposits (1)	45,895,018	44,206,929
Time deposits	41,239	46,885
Commitments to extend credit	21,507	22,208
Estimated Fair Value | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		700,000
Estimated Fair Value | Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings		330,000
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	1,102,140	3,730
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	343,676	352,058
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	369,005	374,483
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,619,384	2,923,075
Held-to-maturity securities	0	0
Non-marketable securities not measured at net asset value	0	0
FHLB and Federal Reserve Bank stock	0	0
Non-maturity deposits (1)	45,895,018	44,206,929
Time deposits	0	0
Commitments to extend credit	0	0
Estimated Fair Value | Level 1 | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		700,000
Estimated Fair Value | Level 1 | Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings		330,000
Estimated Fair Value | Level 1 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	1,102,140	3,730
Estimated Fair Value | Level 1 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Estimated Fair Value | Level 1 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity securities	14,229,439	12,548,280
Non-marketable securities not measured at net asset value	0	0
FHLB and Federal Reserve Bank stock	0	0
Non-maturity deposits (1)	0	0
Time deposits	41,239	46,885
Commitments to extend credit	0	0
Estimated Fair Value | Level 2 | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		0
Estimated Fair Value | Level 2 | Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings		0
Estimated Fair Value | Level 2 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	0	0
Estimated Fair Value | Level 2 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	343,676	352,058
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	369,005	374,483
Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity securities	0	0
Non-marketable securities not measured at net asset value	123,811	126,345
FHLB and Federal Reserve Bank stock	60,514	60,020
Non-maturity deposits (1)	0	0
Time deposits	0	0
Commitments to extend credit	21,507	22,208
Estimated Fair Value | Level 3 | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		0
Estimated Fair Value | Level 3 | Federal Funds Purchased [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings		0
Estimated Fair Value | Level 3 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	0	0
Estimated Fair Value | Level 3 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0
Estimated Fair Value | Level 3 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Consumer Portfolio Segment [Member] | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,695,031	2,613,045
Consumer Portfolio Segment [Member] | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,695,873	2,593,538
Consumer Portfolio Segment [Member] | Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Consumer Portfolio Segment [Member] | Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Consumer Portfolio Segment [Member] | Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,695,873	2,593,538
Commercial Portfolio Segment | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	21,618,619	20,238,247
Commercial Portfolio Segment | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	21,810,467	20,520,623
Commercial Portfolio Segment | Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Commercial Portfolio Segment | Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Commercial Portfolio Segment | Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	21,810,467	$ 20,520,623
Fair Value, Measurements, Recurring | Other derivative instruments | Other liability | Derivatives not designated as hedging instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Liabilities	(13,369)
Fair Value, Measurements, Recurring | Other derivative instruments | Other liability | Derivatives not designated as hedging instruments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Liabilities	0
Fair Value, Measurements, Recurring | Other derivative instruments | Other liability | Derivatives not designated as hedging instruments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Liabilities	(13,369)
Fair Value, Measurements, Recurring | Other derivative instruments | Other liability | Derivatives not designated as hedging instruments | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Liabilities	$ 0