Fair Value of Financial Instruments Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity Securities, Fair Value	$ 12,548,280	$ 8,376,138
Net loans	22,851,292	19,674,578
Short-term borrowings	1,033,730	512,668
Time Deposits	47,146	56,118
Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	330,000	0
Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Home Loan Bank Advances	700,000	500,000
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,730	12,668
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,303	346,979
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,189	347,586
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	0	46,646
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	0	54,493
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,923,075	2,545,750
Held-to-maturity Securities, Fair Value	12,663,455	8,426,998
Non-marketable securities (cost and equity method accounting)	120,019	120,037
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Non-Marketable Securities	228,399	245,626
FHLB and FRB stock	60,020	57,592
Accrued interest receivable	141,773	111,222
Non-maturity deposits (2)	44,206,929	38,923,750
Time Deposits	47,146	56,118
Accrued interest payable	11,192	12,013
Commitments to extend credit	0	0
Carrying Amount | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	330,000
Carrying Amount | Federal Home Loan Bank Advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Federal Home Loan Bank Advances	700,000	500,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,730	12,668
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,303	346,979
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,189	347,586
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes		46,646
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures		54,493
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,923,075	2,545,750
Held-to-maturity Securities, Fair Value	12,548,280	8,376,138
Non-marketable securities (cost and equity method accounting)	126,345	127,343
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Non-Marketable Securities	331,496	353,870
FHLB and FRB stock	60,020	57,592
Accrued interest receivable	141,773	111,222
Federal Home Loan Bank Advances	700,000	500,000
Non-maturity deposits (2)	44,206,929	38,923,750
Time Deposits	46,885	55,949
Accrued interest payable	11,192	12,013
Commitments to extend credit	22,208	22,074
Estimated Fair Value | Federal Funds Purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	330,000
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,730	12,668
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	352,058	337,600
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	374,483	378,777
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes		47,489
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures		53,140
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	2,923,075	2,545,750
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)	44,206,929	38,923,750
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	330,000
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	700,000	500,000
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	3,730	12,668
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
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
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	12,548,280	8,376,138
Non-marketable securities (cost and equity method accounting)	0	0
FHLB and FRB stock	0	0
Accrued interest receivable	141,773	111,222
Non-maturity deposits (2)	0	0
Time Deposits	46,885	55,949
Accrued interest payable	11,192	12,013
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	352,058	337,600
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	374,483	378,777
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
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
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)	126,345	127,343
FHLB and FRB stock	60,020	57,592
Accrued interest receivable	0	0
Non-maturity deposits (2)	0	0
Time Deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	22,208	22,074
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
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
Commercial loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	20,238,247	17,518,430
Commercial loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	20,520,623	17,811,356
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	20,520,623	17,811,356
Consumer loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	2,613,045	2,156,148
Consumer loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	2,593,538	2,199,501
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	$ 2,593,538	$ 2,199,501