Derivatives and Hedging Activities - Interest Rate Derivatives Outstanding Designated as Cash Flow Hedges (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Interest Rate Derivatives Outstanding [Line Items]
Notional	$ 7,750,000
Fixed Interest Rate in Contract	2.14%
Bank of New York Mellon [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	May 24, 2013
Fixed Interest Rate in Contract	1.83%
Bank of New York Mellon [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	200,000
Maturity Date	Jun. 15, 2013
Fixed Interest Rate in Contract	1.73%
SunTrust Bank [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	Jul. 15, 2014
Fixed Interest Rate in Contract	2.79%
Deutsche Bank AG [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	200,000
Maturity Date	Jan. 15, 2015
Fixed Interest Rate in Contract	1.08%
Deutsche Bank AG [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	250,000
Maturity Date	Feb. 15, 2015
Fixed Interest Rate in Contract	1.14%
Credit Suisse International [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	Feb. 24, 2015
Fixed Interest Rate in Contract	3.26%
Credit Suisse International [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	Mar. 24, 2015
Fixed Interest Rate in Contract	2.76%
Wells Fargo Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	Jul. 15, 2015
Fixed Interest Rate in Contract	2.85%
Wells Fargo Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	50,000
Maturity Date	Jul. 15, 2015
Fixed Interest Rate in Contract	2.44%
Morgan Stanley Capital Services, Inc. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	300,000
Maturity Date	Jan. 24, 2016
Fixed Interest Rate in Contract	2.12%
Bank of New York Mellon [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	300,000
Maturity Date	Jan. 24, 2016
Fixed Interest Rate in Contract	2.13%
Morgan Stanley Capital Services, Inc. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	300,000
Maturity Date	Apr. 05, 2016
Fixed Interest Rate in Contract	2.48%
Citibank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	300,000
Maturity Date	Apr. 15, 2016
Fixed Interest Rate in Contract	1.67%
Bank of New York Mellon [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	Apr. 15, 2016
Fixed Interest Rate in Contract	2.24%
Credit Suisse International [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	Apr. 15, 2016
Fixed Interest Rate in Contract	2.27%
JPMorgan Chase Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	May 16, 2016
Fixed Interest Rate in Contract	2.31%
Goldman Sachs Bank USA [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	May 24, 2016
Fixed Interest Rate in Contract	2.34%
Wells Fargo Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	250,000
Maturity Date	Jun. 15, 2016
Fixed Interest Rate in Contract	2.67%
Goldman Sachs Bank USA [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	250,000
Maturity Date	Jun. 15, 2016
Fixed Interest Rate in Contract	2.67%
JPMorgan Chase Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	Jun. 24, 2016
Fixed Interest Rate in Contract	2.51%
Citibank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	Oct. 15, 2016
Fixed Interest Rate in Contract	1.93%
Deutsche Bank AG [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	150,000
Maturity Date	Feb. 05, 2018
Fixed Interest Rate in Contract	2.90%
Morgan Stanley Capital Services, Inc. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	100,000
Maturity Date	Apr. 05, 2018
Fixed Interest Rate in Contract	3.10%
JPMorgan Chase Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	200,000
Maturity Date	May 15, 2018
Fixed Interest Rate in Contract	2.93%
Ubs Ag May Twenty Fourth Two Thousand Eighteen [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	May 24, 2018
Fixed Interest Rate in Contract	1.10%
Royal Bank of Scotland Plc September Fifth Two Thousand Eighteen [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	500,000
Maturity Date	Sep. 05, 2018
Fixed Interest Rate in Contract	1.04%
Wells Fargo Bank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	200,000
Maturity Date	Mar. 15, 2021
Fixed Interest Rate in Contract	3.14%
Citibank, N.A. [Member]
Interest Rate Derivatives Outstanding [Line Items]
Notional	$ 200,000
Maturity Date	May 25, 2021
Fixed Interest Rate in Contract	2.83%