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	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 7,869,653		$ 7,415,656
Federal Home Loan Bank Advances	60,000		0
Short-term borrowings	77,766		7,781
Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	17,766	[1]	7,781	[1]
Senior Notes Three Point Five Zero Percent Due January Twenty Twenty Five [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	349,710		0
5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,495		348,436
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,910	[2]	50,162	[2]
7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,802		54,845
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,308,003		1,796,062
Held-to-maturity securities	7,816,797		7,421,042
Non-marketable securities (cost and equity method accounting)	295,427		296,648
Net commercial loans	12,900,215		12,947,869
Net consumer loans	1,371,484		1,271,048
Assets Held-for-sale, Long Lived, Fair Value Disclosure	48,200		44,253
FHLB and Federal Reserve Bank stock	53,496		53,496
Accrued interest receivable	91,415		94,180
Non-maturity deposits	33,752,797	[3]	34,215,372	[3]
Time deposits	98,778		128,127
Accrued interest payable	5,038		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	17,766		7,781
Carrying Amount | Senior Notes Three Point Five Zero Percent Due January Twenty Twenty Five [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	349,710
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,495		348,436
Carrying Amount | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	49,910	[4]	50,162	[4]
Carrying Amount | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	54,802		54,845
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets Held-for-sale, Long Lived, Fair Value Disclosure			45,410
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,308,003		1,796,062
Held-to-maturity securities	0		0
Non-marketable 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
Federal Home Loan Bank Advances	60,000
Non-maturity deposits	33,752,797	[3]	34,215,372	[3]
Time deposits	0		0
Senior Notes	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	17,766		7,781
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	7,869,653		7,415,656
Non-marketable 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	91,415		94,180
Federal Home Loan Bank Advances	0
Non-maturity deposits	0	[3]	0	[3]
Time deposits	98,740		128,107
Accrued interest payable	5,038		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 | Senior Notes Three Point Five Zero Percent Due January Twenty Twenty Five [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,639
Estimated Fair Value | Level 2 | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	396,799		392,616
Estimated Fair Value | Level 2 | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Subordinated Notes	52,963	[4]	53,537	[4]
Estimated Fair Value | Level 2 | 7.0% Junior Subordinated Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Junior Subordinated Debentures	53,109		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)	402,301		390,570
Net commercial loans	13,034,516		13,082,487
Net consumer loans	1,347,284		1,247,336
FHLB and Federal Reserve Bank stock	53,496		53,496
Accrued interest receivable	0		0
Federal Home Loan Bank Advances	0
Non-maturity deposits	0	[3]	0	[3]
Time deposits	0		0
Senior Notes	0
Accrued interest payable	0		0
Commitments to extend credit	30,135		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 | 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, which includes an interest rate swap agreement related to our 6.05% Subordinated Notes.

[2] At March 31, 2015 and December 31, 2014, included in the carrying value of our 6.05% Subordinated Notes was an interest rate swap valued at $4.3 million and $4.6 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 March 31, 2015 and December 31, 2014, included in the carrying value and estimated fair value of our 6.05% Subordinated Notes was an interest rate swap valued at $4.3 million and $4.6 million, respectively, related to hedge accounting associated with the notes.