Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 15,372,238	$ 12,548,280
Loans and Leases Receivable, Net Amount	27,209,202	22,851,292
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,819,141	2,923,075
Held-to-maturity securities	15,899,726	12,663,455
Non-marketable securities not measured at net asset value	134,646	120,019
Non-marketable securities measured at net asset value	134,017	228,399
FHLB and Federal Reserve Bank stock	102,378	60,020
Non-maturity deposits	48,518,701	44,206,929
Time deposits	77,410	47,146
Commitments to extend credit	0	0
Carrying Amount | Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	2,631,252	1,033,730
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,554	347,303
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	348,663	348,189
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,819,141	2,923,075
Held-to-maturity securities	15,372,238	12,548,280
Non-marketable securities not measured at net asset value	134,646	126,345
Non-marketable securities measured at net asset value	134,017	331,496
FHLB and Federal Reserve Bank stock	102,378	60,020
Non-maturity deposits	48,518,701	44,206,929
Time deposits	77,047	46,885
Commitments to extend credit	22,857	22,208
Estimated Fair Value | Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	2,631,252	1,033,730
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	338,370	352,058
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	363,447	374,483
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,819,141	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	48,518,701	44,206,929
Time deposits	0	0
Commitments to extend credit	0	0
Estimated Fair Value | Level 1 | Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	1,033,730
Estimated Fair Value | Level 1 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	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	15,372,238	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	0	0
Time deposits	77,047	46,885
Commitments to extend credit	0	0
Estimated Fair Value | Level 2 | Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	2,631,252	0
Estimated Fair Value | Level 2 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	338,370	352,058
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	363,447	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	134,646	126,345
FHLB and Federal Reserve Bank stock	102,378	60,020
Non-maturity deposits	0	0
Time deposits	0	0
Commitments to extend credit	22,857	22,208
Estimated Fair Value | Level 3 | Short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
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	0
Estimated Fair Value | Level 3 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	0	0
Commercial loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	24,253,625	20,238,247
Commercial loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	23,285,628	20,520,623
Commercial loans | Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Commercial loans | Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Commercial loans | Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	23,285,628	20,520,623
Consumer loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,955,577	2,613,045
Consumer loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,809,916	2,593,538
Consumer loans | Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Consumer loans | Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	0	0
Consumer loans | Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	$ 2,809,916	$ 2,593,538