Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 01, 2014	Dec. 31, 2013	Jun. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Transfers from available-for-sale securities to held-to-maturity	$ 5,463,920	$ 5,418,572		$ 0
Held-to-maturity Securities, Fair Value	5,454,996		0
Short-term borrowings	4,910		5,080
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	4,910		5,080
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,321		348,209
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	51,208		51,987
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,933		55,020
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,649,831		1,538,779
Non-marketable and other securities (cost and equity method accounting)	391,327		378,309
Net commercial loans	10,127,781		9,796,878
Net consumer loans	1,100,202		966,622
FHLB and Federal Reserve Bank stock	41,026		40,632
Accrued interest receivable	79,135		67,772
Non-maturity deposits	28,196,740		22,259,119
Time deposits	155,807		213,860
Accrued interest payable	6,895		6,858
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	4,910		5,080
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,321		348,209
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	51,208		51,987
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,933		55,020
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,649,831		1,538,779
Held-to-maturity Securities, Fair Value	0
Non-marketable and other securities (cost and equity method accounting)	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	28,196,740		22,259,119
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	4,910		5,080
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, Fair Value	5,454,996
Non-marketable and other securities (cost and equity method accounting)	0		0
Net commercial loans	0		0
Net consumer loans	0		0
FHLB and Federal Reserve Bank stock	0		0
Accrued interest receivable	79,135		67,772
Non-maturity deposits	0		0
Time deposits	155,812		213,874
Accrued interest payable	6,895		6,858
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 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	396,452		383,782
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	55,806		56,297
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,289		51,915
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, Fair Value	0
Non-marketable and other securities (cost and equity method accounting)	480,736		447,783
Net commercial loans	10,319,273		9,935,917
Net consumer loans	1,039,405		1,005,080
FHLB and Federal Reserve Bank stock	41,026		40,632
Accrued interest receivable	0		0
Non-maturity deposits	0		0
Time deposits	0		0
Accrued interest payable	0		0
Commitments to extend credit	27,698		24,285
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 | 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