CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 376.8	$ 345.9
Foreign currency translation:
Foreign currency translation adjustments - Pre Tax	121.6	14.4
Foreign currency translation adjustment - Tax	3.6	(2.3)
Foreign currency translation adjustments - Net of Tax	125.2	12.1
Cash flow hedges:
Net realized and unrealized gain (loss) on cash flow hedges - Pre Tax	1.9	(0.3)
Net realized and unrealized gain (loss) on cash flow hedges - Tax Amount	(0.4)	0.1
Net realized and unrealized gain (loss) on cash flow hedges - Net of Tax	1.5	(0.2)
Reclassification of losses included in net income - Pre Tax	(0.1)	(1.4)
Reclassification of losses included in net income - Tax Amount		0.5
Reclassification of losses included in net income- Net of Tax	(0.1)	(0.9)
Available for sale securities:
Net unrealized gains on available for sale securities - Pre Tax		0.5
Net unrealized gains on available for sale securities - Net of Tax		0.5
Pension and Other Retirement Benefits:
Amortization of actuarial losses and prior service costs included in net income - Pre Tax	1.4	2.4
Amortization of actuarial losses and prior service costs included in net income - Tax	(0.4)	(0.9)
Amortization of actuarial losses and prior service costs included in net income - Net of Tax	1.0	1.5
Total other comprehensive income (loss) - Pre Tax	124.8	15.6
Total other comprehensive income (loss) - Tax	2.8	(2.6)
Total other comprehensive income (loss) - Net of Tax	127.6	13.0
Comprehensive income	504.4	358.9
Less: Comprehensive income attributable to noncontrolling interests and redeemable noncontrolling interest	8.9	5.7
Comprehensive income attributable to Moody's	$ 495.5	$ 353.2