CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments Gain Loss [Line Items]
Net income	$ 377.9	$ 313.6	$ 754.7	$ 659.5
Foreign currency translation:
Foreign currency translation adjustments - Pre Tax	(250.3)	35.1	(128.7)	49.5
Foreign currency translation adjustment - Tax	(9.2)	15.4	(5.6)	13.1
Foreign currency translation adjustments - Net of Tax	(259.5)	50.5	(134.3)	62.6
Cash flow hedges:
Net realized and unrealized gain (loss) on cash flow hedges - Net of Tax	27.4	(13.5)	18.0	(17.3)
Reclassification of losses included in net income - Pre Tax	(0.1)	(6.1)	(0.2)	(7.5)
Reclassification of losses included in net income - Tax Amount		2.8		3.3
Reclassification of losses included in net income- Net of Tax	(0.1)	(3.3)	(0.2)	(4.2)
Available for sale securities:
Net unrealized gains on available for sale securities - Pre Tax		0.6		1.1
Net unrealized gains on available for sale securities - Net of Tax		0.6		1.1
Pension and Other Retirement Benefits:
Amortization of actuarial losses and prior service costs included in net income - Pre Tax	1.0	1.9	2.4	4.3
Amortization of actuarial losses and prior service costs included in net income - Tax	(0.3)	(0.8)	(0.7)	(1.7)
Amortization of actuarial losses and prior service costs included in net income - Net of Tax	0.7	1.1	1.7	2.6
Net actuarial losses and prior service costs - Pre Tax	1.6	7.9	1.6	7.9
Net actuarial losses and prior service costs - Tax	(0.4)	(3.0)	(0.4)	(3.0)
Net actuarial losses and prior service costs - Net of Tax	1.2	4.9	1.2	4.9
Total other comprehensive (loss) income - Pre Tax	(247.8)	44.5	(123.0)	60.1
Total other comprehensive (loss)income - Tax	(9.9)	12.5	(7.1)	9.9
Total other comprehensive (loss) income - Net of Tax	(257.7)	57.0	(130.1)	70.0
Comprehensive income	120.2	370.6	624.6	729.5
Less: Comprehensive income attributable to noncontrolling interests and redeemable noncontrolling interest	(3.2)	10.8	5.7	16.5
Comprehensive income attributable to Moody's	123.4	359.8	618.9	713.0
Cash Flow Hedging [Member]
Cash flow hedges:
Net realized and unrealized gain (loss) on cash flow hedges - Pre Tax		5.1	1.9	4.8
Net realized and unrealized gain (loss) on cash flow hedges - Tax Amount		(1.9)	(0.4)	(1.8)
Net realized and unrealized gain (loss) on cash flow hedges - Net of Tax		3.2	1.5	3.0
Reclassification of losses included in net income- Net of Tax	$ (0.1)	$ (3.3)	$ (0.2)	$ (4.2)