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	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 6,613,893		$ 0
Short-term borrowings	6,630		5,080
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	6,630	[1]	5,080	[1]
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,378		348,209
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	50,497	[2]	51,987	[2]
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,889		55,020
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,872,537		1,538,779
Held-to-maturity securities	6,662,025
Non-marketable and other securities (cost and equity method accounting)	399,511		378,309
Net commercial loans	10,721,645		9,796,878
Net consumer loans	1,166,475		966,622
FHLB and Federal Reserve Bank stock	41,026		40,632
Accrued interest receivable	81,334		67,772
Non-maturity deposits	30,963,491	[3]	22,259,119	[3]
Time deposits	159,644		213,860
Accrued interest payable	2,932		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	6,630		5,080
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,378		348,209
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	50,497	[4]	51,987	[4]
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,889		55,020
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,872,537		1,538,779
Held-to-maturity securities	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	30,963,491	[3]	22,259,119	[3]
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	6,630		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	[4]	0	[4]
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	6,613,893
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	81,334		67,772
Non-maturity deposits	0	[3]	0	[3]
Time deposits	159,644		213,874
Accrued interest payable	2,932		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	390,740		383,782
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	54,423	[4]	56,297	[4]
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	52,722		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	0
Non-marketable and other securities (cost and equity method accounting)	497,855		447,783
Net commercial loans	10,884,474		9,935,917
Net consumer loans	1,121,639		1,005,080
FHLB and Federal Reserve Bank stock	41,026		40,632
Accrued interest receivable	0		0
Non-maturity deposits	0	[3]	0	[3]
Time deposits	0		0
Accrued interest payable	0		0
Commitments to extend credit	29,731		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	[4]	0	[4]
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

[1] Represents cash collateral received from certain counterparties in relation to market value exposures of derivative contracts in our favor, primarily for our interest rate swap agreement related to our 6.05% Subordinated Notes.

[2] At September 30, 2014 and December 31, 2013, included in the carrying value of our 6.05% Subordinated Notes was an interest rate swap valued at $5.0 million and $6.5 million, respectively, related to hedge accounting associated with the notes.

[3] Includes noninterest-bearing demand deposits, interest-bearing checking accounts, money market accounts and interest-bearing sweep deposits.

[4] At September 30, 2014 and December 31, 2013, included in the carrying value and estimated fair value of our 6.05% Subordinated Notes was an interest rate swap valued at $5.0 million and $6.5 million, respectively, related to hedge accounting associated with the notes.