Restructuring - Summary (Details) - 2017 Restructuring Plans $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) position
Restructuring Cost and Reserve [Line Items]
Workforce reduction | position		520
Initial Charge Recorded	$ 44
Ending Reserve Balance	33	$ 39
Restructuring charges paid	6
Operating Segments | Ratings
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	25
Ending Reserve Balance	21
Operating Segments | Market Intelligence
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	8
Ending Reserve Balance	4
Operating Segments | Platts
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	1
Ending Reserve Balance	0
Operating Segments | Indices
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	0
Ending Reserve Balance	0
Corporate
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	10
Ending Reserve Balance	$ 8