Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 8,630,952	$ 8,758,622
Federal funds purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	135,000
3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,744	346,667
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,155	347,016
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	48,045	48,350
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,626	54,669
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,868,512	1,503,257
Held-to-maturity securities	8,548,238	8,790,963
Non-marketable securities (cost and equity method accounting) not measured at net asset value	117,471	114,795
Non-marketable securities (cost and equity method accounting) measured at net asset value (1)	245,125	250,970
Net commercial loans	15,633,705	14,763,302
Net consumer loans	1,871,193	1,761,155
FHLB and Federal Reserve Bank stock	56,991	56,991
Accrued interest receivable	105,062	107,604
Non-maturity deposits	38,695,925	39,072,297
Time deposits	63,796	70,479
Accrued interest payable	4,888	12,058
Commitments to extend credit	0	0
Carrying Amount | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		638,000
Carrying Amount | Federal funds purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		135,000
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		1,900
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	346,744	346,667
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,155	347,016
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	48,045	48,350
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,626	54,669
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,868,512	1,503,257
Held-to-maturity securities	8,630,952	8,758,622
Non-marketable securities (cost and equity method accounting) not measured at net asset value	120,299	117,172
Non-marketable securities (cost and equity method accounting) measured at net asset value (1)	361,657	364,799
Net commercial loans	15,703,446	14,811,588
Net consumer loans	1,847,619	1,737,960
FHLB and Federal Reserve Bank stock	56,991	56,991
Accrued interest receivable	105,062	107,604
Non-maturity deposits	38,695,925	39,072,297
Time deposits	63,686	70,347
Accrued interest payable	4,888	12,058
Commitments to extend credit	25,515	26,483
Estimated Fair Value | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		638,000
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		1,900
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	345,506	333,648
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	389,946	384,216
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,368	49,820
Estimated Fair Value | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	53,039	52,905
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,868,512	1,503,257
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	38,695,925	39,072,297
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 borrowings		638,000
Estimated Fair Value | Level 1 | Federal funds purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		135,000
Estimated Fair Value | Level 1 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		1,900
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	8,630,952	8,758,622
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	105,062	107,604
Non-maturity deposits	0	0
Time deposits	63,686	70,347
Accrued interest payable	4,888	12,058
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 borrowings		0
Estimated Fair Value | Level 2 | Federal funds purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		0
Estimated Fair Value | Level 2 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		0
Estimated Fair Value | Level 2 | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	345,506	333,648
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	389,946	384,216
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,368	49,820
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	53,039	52,905
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	120,299	117,172
Net commercial loans	15,703,446	14,811,588
Net consumer loans	1,847,619	1,737,960
FHLB and Federal Reserve Bank stock	56,991	56,991
Accrued interest receivable	0	0
Non-maturity deposits	0	0
Time deposits	0	0
Accrued interest payable	0	0
Commitments to extend credit	25,515	26,483
Estimated Fair Value | Level 3 | Short-term FHLB advances
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		0
Estimated Fair Value | Level 3 | Federal funds purchased
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		0
Estimated Fair Value | Level 3 | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings		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