Comprehensive Income And Noncontrolling Interests (Components Of Total Comprehensive Income, Net Of Tax) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive Income And Noncontrolling Interests [Line Items]
Net income	$ 132.1	$ 137.2	$ 479.9	$ 374.5
Net realized and unrealized gain/(loss) on cash flow hedges	0.7	(0.6)	1.8	(0.3)
FX translation	(74.4)	59.5	(38.2)	11.8
Net actuarial gains and prior service costs			(3.3)	4.1
Amortization and recognition of actuarial losses and prior service costs	0.6	0.5	2.3	1.3
Total comprehensive income	59.0	196.6	442.5	391.4
Net realized and unrealized gain/(loss) on cash flow hedges, tax	0.5	0.1	1.2	0.1
FX translation, tax	0.1	1.7	0.1	10.3
Net actuarial gains and prior service costs, tax			2.4	2.9
Amortization and recognition of actuarial losses and prior service costs, tax	0.5	0.4	1.7	1.4
Shareholders' Of Moody's Corporation [Member]
Comprehensive Income And Noncontrolling Interests [Line Items]
Net income	130.7	136.0	475.2	370.4
Net realized and unrealized gain/(loss) on cash flow hedges	0.7	(0.6)	1.8	(0.3)
FX translation	(73.3)	58.8	(37.9)	11.5
Net actuarial gains and prior service costs			(3.3)	4.1
Amortization and recognition of actuarial losses and prior service costs	0.6	0.5	2.3	1.3
Total comprehensive income	58.7	194.7	438.1	387.0
Noncontrolling Interests [Member]
Comprehensive Income And Noncontrolling Interests [Line Items]
Net income	1.4	1.2	4.7	4.1
FX translation	(1.1)	0.7	(0.3)	0.3
Total comprehensive income	$ 0.3	$ 1.9	$ 4.4	$ 4.4