Revenue Recognition Timing (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue Recognized	$ 1,126.7
At Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	478.5
Over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	648.2
Moodys Investors Service [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	719.9
Moodys Investors Service [Member] | At Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	463.0
Moodys Investors Service [Member] | Over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	256.9
Moodys Analytics [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	406.8
Moodys Analytics [Member] | At Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	15.5
Moodys Analytics [Member] | Over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue Recognized	$ 391.3