Indebtedness (Narrative) (Details) (USD $)	9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2011	May 31, 2008	May 07, 2008	Sep. 30, 2011 Line Of Credit [Member]	Sep. 28, 2007 Line Of Credit [Member]	Sep. 30, 2011 Commercial Paper [Member]	Oct. 03, 2007 Commercial Paper [Member]	Sep. 30, 2011 2010 Senior Notes [Member]	Sep. 30, 2011 Series 2007-1 Notes [Member]	Sep. 30, 2011 Series 2005-1 Notes [Member]	Sep. 30, 2011 Senior Notes 7.61 Percent Due September 30th 2005 [Member]	Sep. 30, 2011 2008 Term Loan [Member]
Debt Instrument [Line Items]
Date the credit facility was entered				2007-09-28
Date of expiry of credit facility				2012-09-28
Financial covenants to be maintained at the end of any fiscal quarter				Debt/EBITDA ratio of not more than 4.0 to 1.0
Five-year senior, unsecured revolving credit facility					$ 1,000,000,000		$ 1,000,000,000
Maturity date of CP Notes						397 days
Interest on borrowings under the 2008 Term Loan				LIBOR plus a premium that can range from 16.0 to 40.0 basis points								LIBOR plus a margin that can range from 125 basis points to 175 basis points
Quarterly facility fees				4.0 to 10.0 basis points per annum
Utilization fee				5.0 basis points on borrowings outstanding when the aggregate amount outstanding exceeds 50% of the total facility
Debt to EBITDA ratio not to exceed at the end of any fiscal quarter									Debt/EBITDA ratio to exceed 4.0 to 1.0 at the end of any fiscal quarter			Debt/EBITDA ratio of not more than 4.0 to 1.0
Date of agreement with the lenders for the five year senior unsecured term loan								2010-08-19	September 7, 2007	2005-09-30		May 7, 2008
Entered with several lenders into a five year senior unsecured term loan/Refinance of Senior Notes								500,000,000	300,000,000	300,000,000	300,000,000	150,000,000
Minimum amount for default on senior notes payable	50,000,000
Minimum percentage for default on senior notes payable	Upon the occurrence and during the continuation of an event of default under the Indenture, the notes may become immediately due and payable either automatically or by the vote of the holders of more than 25% of the aggregate principal amount of all of the notes then outstanding
Senior Unsecured Notes, interest								5.50%	6.06%	4.98%	7.61%
Maturity date of Senior Unsecured Notes								Sep. 01, 2020	Sep. 07, 2017	Sep. 30, 2015
Prepayment and purchase feature of Senior Unsecured Notes	In the event that Moody's pays all, or part, of the Series 2005-1 Notes in advance of their maturity, such prepayment will be subject to a Make Whole Amount							The Company may prepay the 2010 Senior Notes, in whole or in part, at any time at a price equal to 100% of the principal amount being prepaid, plus accrued and unpaid interest and a Make-Whole Amount. Additionally, at the option of the holders of the notes, the Company may be required to purchase all or a portion of the notes upon occurrence of a "Change of Control Triggering Event," as defined in the Indenture, at a price equal to 101% of the principal amount thereof, plus accrued and unpaid interest to the date of purchase	The Company may prepay the Series 2007-1 Notes, in whole or in part, at any time at a price equal to 100% of the principal amount being prepaid, plus accrued and unpaid interest and a Make Whole Amount
Issuance of additional principal amount of Senior Notes within five years from time to time									500,000,000
Interest rate swaps total notional amount		$ 150,000,000	$ 150,000,000							$ 300,000,000
Debt instrument, term (years)									10	10