Credit Facility - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Unsecured Revolving Line Of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unsecured	$ 350.0
Line of credit facility, Maturity date	Dec. 18, 2012
Syndicated Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unsecured	850.0
Line of credit facility, Expiration date	Sep. 11, 2017
Line of Credit Facility Amount of Option to Increase Additional Borrowings	$ 350.0
Line of credit facility, initial margin rate	0.75%