Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 8,567,817	$ 8,376,138
Loans and Leases Receivable, Net Amount	20,184,321	19,674,578
Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances	0	500,000
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	5,163	12,668
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,059	346,979
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,733	347,586
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	46,223	46,646
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	54,450	54,493
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,795,679	2,545,750
Held-to-maturity securities	8,615,695	8,426,998
Non-marketable securities (cost and equity method accounting) not measured at net asset value	121,108	120,037
Non-marketable securities (cost and equity method accounting) measured at net asset value	239,275	245,626
FHLB and Federal Reserve Bank stock	57,592	57,592
Accrued interest receivable	110,949	111,222
Non-maturity deposits (1)	41,038,552	38,923,750
Time deposits	41,148	56,118
Accrued interest payable	4,990	12,013
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		500,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	5,163	12,668
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,059	346,979
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	347,733	347,586
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	46,223	46,646
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	54,450	54,493
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,795,679	2,545,750
Held-to-maturity securities	8,567,817	8,376,138
Non-marketable securities (cost and equity method accounting) not measured at net asset value	125,650	127,343
Non-marketable securities (cost and equity method accounting) measured at net asset value	346,890	353,870
FHLB and Federal Reserve Bank stock	57,592	57,592
Accrued interest receivable	110,949	111,222
Non-maturity deposits (1)	41,038,552	38,923,750
Time deposits	40,982	55,949
Accrued interest payable	4,990	12,013
Commitments to extend credit	21,127	22,074
Estimated Fair Value | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term FHLB advances		500,000
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	5,163	12,668
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	340,582	337,600
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	379,236	378,777
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	46,587	47,489
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	54,021	53,140
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,795,679	2,545,750
Held-to-maturity securities	0	0
Non-marketable securities (cost and equity method accounting) not measured at net asset value	0	0
FHLB and Federal Reserve Bank stock	0	0
Accrued interest receivable	0	0
Non-maturity deposits (1)	41,038,552	38,923,750
Time deposits	0	0
Accrued interest payable	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		500,000
Estimated Fair Value | Level 1 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other short-term borrowings	5,163	12,668
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 1 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	0	0
Estimated Fair Value | Level 1 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	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	8,567,817	8,376,138
Non-marketable securities (cost and equity method accounting) not measured at net asset value	0	0
FHLB and Federal Reserve Bank stock	0	0
Accrued interest receivable	110,949	111,222
Non-maturity deposits (1)	0	0
Time deposits	40,982	55,949
Accrued interest payable	4,990	12,013
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 | 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	340,582	337,600
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	379,236	378,777
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	46,587	47,489
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	54,021	53,140
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 (cost and equity method accounting) not measured at net asset value	125,650	127,343
FHLB and Federal Reserve Bank stock	57,592	57,592
Accrued interest receivable	0	0
Non-maturity deposits (1)	0	0
Time deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	21,127	22,074
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 | 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
Estimated Fair Value | Level 3 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated notes	0	0
Estimated Fair Value | Level 3 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior subordinated debentures	0	0
Consumer Portfolio Segment [Member] | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,231,070	2,156,148
Consumer Portfolio Segment [Member] | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	2,267,387	2,199,501
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	2,267,387	2,199,501
Commercial Portfolio Segment | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	17,953,251	17,518,430
Commercial Portfolio Segment | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and Leases Receivable, Net Amount	18,165,969	17,811,356
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	18,165,969	$ 17,811,356
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	(10,592)
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	(10,592)
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