Fair Value of Financial Instruments Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity Securities, Fair Value	$ 8,376,138	$ 8,758,622
Net loans	19,674,578	16,524,457
Short-term borrowings	512,668	774,900
Time Deposits	56,118	70,479
Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	135,000
Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Home Loan Bank Advances	500,000	638,000
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	12,668	1,900
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,979	346,667
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,586	347,016
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	46,646	48,350
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,493	54,669
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,545,750	1,503,257
Held-to-maturity Securities, Fair Value	8,426,998	8,790,963
Non-marketable securities (cost and equity method accounting)	120,037	114,795
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Non-Marketable Securities	245,626	250,970
FHLB and FRB stock	57,592	56,991
Accrued interest receivable	111,222	107,604
Non-maturity deposits (2)	38,923,750	39,072,297
Time Deposits	56,118	70,479
Accrued interest payable	12,013	12,058
Commitments to extend credit	0	0
Carrying Amount | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		135,000
Carrying Amount | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Home Loan Bank Advances	500,000	638,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	12,668	1,900
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,979	346,667
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,586	347,016
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	46,646	48,350
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,493	54,669
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,545,750	1,503,257
Held-to-maturity Securities, Fair Value	8,376,138	8,758,622
Non-marketable securities (cost and equity method accounting)	127,343	117,172
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Non-Marketable Securities	353,870	364,799
FHLB and FRB stock	57,592	56,991
Accrued interest receivable	111,222	107,604
Federal Home Loan Bank Advances	500,000	638,000
Non-maturity deposits (2)	38,923,750	39,072,297
Time Deposits	55,949	70,347
Accrued interest payable	12,013	12,058
Commitments to extend credit	22,074	26,483
Estimated Fair Value | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		135,000
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	12,668	1,900
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	337,600	333,648
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	378,777	384,216
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	47,489	49,820
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	53,140	52,905
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,545,750	1,503,257
Held-to-maturity Securities, Fair Value	0	0
Non-marketable securities (cost and equity method accounting)	0	0
FHLB and FRB stock	0	0
Accrued interest receivable	0	0
Non-maturity deposits (2)	38,923,750	39,072,297
Time Deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	0	0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		135,000
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Home Loan Bank Advances	500,000	638,000
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	12,668	1,900
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0	0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	0	0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (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 | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity Securities, Fair Value	8,376,138	8,758,622
Non-marketable securities (cost and equity method accounting)	0	0
FHLB and FRB stock	0	0
Accrued interest receivable	111,222	107,604
Non-maturity deposits (2)	0	0
Time Deposits	55,949	70,347
Accrued interest payable	12,013	12,058
Commitments to extend credit	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	337,600	333,648
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	378,777	384,216
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	47,489	49,820
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	53,140	52,905
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity Securities, Fair Value	0	0
Non-marketable securities (cost and equity method accounting)	127,343	117,172
FHLB and FRB stock	57,592	56,991
Accrued interest receivable	0	0
Non-maturity deposits (2)	0	0
Time Deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	22,074	26,483
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	0	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	0	0
Commercial loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	17,518,430	14,763,302
Commercial loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	17,811,356	14,811,588
Commercial loans | Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Commercial loans | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Commercial loans | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	17,811,356	14,811,588
Consumer loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	2,156,148	1,761,155
Consumer loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	2,199,501	1,737,960
Consumer loans | Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Consumer loans | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Consumer loans | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	$ 2,199,501	$ 1,737,960