Use of Derivative Financial Instruments, Offsetting Disclosure and Changes in Other Comprehensive Income by Component - Offsetting Disclosures for All Derivatives Held and Repurchase Arrangements and Similar Borrowings Outstanding (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	$ 11,097		$ 32,868
Cash Collateral Pledged	750
Gross Amounts of Recognized Assets	8,396		169
Cash Collateral Received	16,314		49,972
Offsetting Derivatives Assets [Member]
Derivative Instruments And Hedging Activities [Line Items]
Net Amount
Gross Amounts of Recognized Assets	8,396		169
Gross Amounts Offset in the Balance Sheet
Net Amounts of Assets Presented in the Balance Sheet	8,396		169
Financial Instruments	(8,396)		(169)
Cash Collateral Received
Offsetting Derivatives Assets [Member] | Counterparty 1 [Member]
Derivative Instruments And Hedging Activities [Line Items]
Net Amount
Gross Amounts of Recognized Assets	3,536		128
Gross Amounts Offset in the Balance Sheet
Net Amounts of Assets Presented in the Balance Sheet	3,536		128
Financial Instruments	(3,536)		(128)
Cash Collateral Received
Offsetting Derivatives Assets [Member] | Counterparty 2 [Member]
Derivative Instruments And Hedging Activities [Line Items]
Net Amount
Gross Amounts of Recognized Assets	4,860		41
Gross Amounts Offset in the Balance Sheet
Net Amounts of Assets Presented in the Balance Sheet	4,860		41
Financial Instruments	(4,860)		(41)
Cash Collateral Received
Offsetting Financial Liabilities and Derivative Liabilities [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	12,644,536	[1]	12,831,899	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	12,644,536	[1]	12,831,899	[1]
Financial Instruments	(12,635,779)		(12,791,412)
Cash Collateral Pledged	(7,963)		(39,671)
Net Amount	(794)		(816)
Offsetting Financial Liabilities and Derivative Liabilities [Member] | Repurchase Arrangements and Similar Borrowings [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	12,627,383	[1]	12,791,243	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	12,627,383	[1]	12,791,243	[1]
Financial Instruments	(12,627,383)		(12,791,243)
Cash Collateral Pledged
Net Amount
Offsetting Financial Liabilities and Derivative Liabilities [Member] | Counterparty 1 [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	9,918	[1]	26,904	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	9,918	[1]	26,904	[1]
Financial Instruments	(3,536)		(128)
Cash Collateral Pledged	(6,382)		(26,776)
Net Amount
Offsetting Financial Liabilities and Derivative Liabilities [Member] | Counterparty 2 [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	5,654	[1]	12,357	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	5,654	[1]	12,357	[1]
Financial Instruments	(4,860)		(41)
Cash Collateral Pledged			(11,500)
Net Amount	(794)		(816)
Offsetting Financial Liabilities and Derivative Liabilities [Member] | Counterparty 3 [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	1,581	[1]	1,395	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	1,581	[1]	1,395	[1]
Financial Instruments
Cash Collateral Pledged	(1,581)		(1,395)
Net Amount
Offsetting Financial Liabilities and Derivative Liabilities [Member] | Derivative Counterparties [Member]
Derivative Instruments And Hedging Activities [Line Items]
Gross Amounts of Recognized Liabilities	17,153	[1]	40,656	[1]
Gross Amounts Offset in the Balance Sheet
Net Amounts of Liabilities Presented in the Balance Sheet	17,153	[1]	40,656	[1]
Financial Instruments	(8,396)		(169)
Cash Collateral Pledged	(7,963)		(39,671)
Net Amount	$ (794)		$ (816)

[1] Amounts include accrued interest of $6.1 million and $7.8 million on interest rate swap agreements and $2.8 million and $7.0 million on repurchase arrangements and similar borrowings, included in "Accounts payable and accrued expenses" on the face of the Balance Sheets as of June 30, 2013 and December 31, 2012, respectively.