Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 8,367,003	$ 7,415,656
Short-term borrowings	3,756	7,781
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,756	7,781
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,591	0
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,878	346,477
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,028	50,040
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,714	54,845
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,674,145	1,796,062
Held-to-maturity securities	8,306,526	7,421,042
Non-marketable securities (cost and equity method accounting) not measured at net asset value	112,984	108,221
Non-marketable securities (cost and equity method) accounting measured at net asset value (1)	253,210	188,427
Net commercial loans	13,470,144	12,947,869
Net consumer loans	1,646,929	1,271,048
FHLB and Federal Reserve Bank stock	46,116	53,496
Accrued interest receivable	98,453	94,180
Non-maturity deposits	36,977,199	34,215,372
Time deposits	72,218	128,127
Accrued interest payable	4,992	6,998
Commitments to extend credit	0	0
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,756	7,781
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,591
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,878	346,477
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,028	50,040
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,714	54,845
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,674,145	1,796,062
Held-to-maturity securities	8,367,003	7,415,656
Non-marketable securities (cost and equity method accounting) not measured at net asset value	115,359	107,451
Non-marketable securities (cost and equity method) accounting measured at net asset value (1)	374,353	283,119
Net commercial loans	13,547,022	13,082,487
Net consumer loans	1,618,492	1,247,336
FHLB and Federal Reserve Bank stock	46,116	53,496
Accrued interest receivable	98,453	94,180
Non-maturity deposits	36,977,199	34,215,372
Time deposits	72,218	128,107
Accrued interest payable	4,992	6,998
Commitments to extend credit	28,805	29,097
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,756	7,781
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	342,125
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	391,286	392,616
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	51,000	53,537
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	52,557	52,990
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,674,145	1,796,062
Held-to-maturity securities	0	0
Non-marketable securities (cost and equity method accounting) not measured at net asset value	0	0
Net commercial loans	0	0
Net consumer loans	0	0
FHLB and Federal Reserve Bank stock	0	0
Accrued interest receivable	0	0
Non-maturity deposits	36,977,199	34,215,372
Time deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	0	0
Estimated Fair Value | Level 1 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	3,756	7,781
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	8,367,003	7,415,656
Non-marketable securities (cost and equity method accounting) not measured at net asset value	0	0
Net commercial loans	0	0
Net consumer loans	0	0
FHLB and Federal Reserve Bank stock	0	0
Accrued interest receivable	98,453	94,180
Non-maturity deposits	0	0
Time deposits	72,218	128,107
Accrued interest payable	4,992	6,998
Commitments to extend credit	0	0
Estimated Fair Value | Level 2 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
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	342,125
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	391,286	392,616
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	51,000	53,537
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	52,557	52,990
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	115,359	107,451
Net commercial loans	13,547,022	13,082,487
Net consumer loans	1,618,492	1,247,336
FHLB and Federal Reserve Bank stock	46,116	53,496
Accrued interest receivable	0	0
Non-maturity deposits	0	0
Time deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	28,805	29,097
Estimated Fair Value | Level 3 | Other 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
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