Restructuring - Summary (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) position	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) position
2018 Restructuring Plans
Restructuring Cost and Reserve [Line Items]
Workforce reduction | position			70
Initial Charge Recorded			$ 9
Ending Reserve Balance	$ 9		9
2017 Restructuring Plans
Restructuring Cost and Reserve [Line Items]
Workforce reduction | position					520
Initial Charge Recorded			44
Ending Reserve Balance	15		15		$ 39
Restructuring charges paid			24
Operating Segments | 2018 Restructuring Plans | Ratings
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			0
Ending Reserve Balance	0		0
Operating Segments | 2018 Restructuring Plans | Market Intelligence
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			2
Ending Reserve Balance	2		2
Operating Segments | 2018 Restructuring Plans | Platts
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			0
Ending Reserve Balance	0		0
Operating Segments | 2018 Restructuring Plans | Indices
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			0
Ending Reserve Balance	0		0
Operating Segments | 2017 Restructuring Plans | Ratings
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			25
Ending Reserve Balance	10		10
Operating Segments | 2017 Restructuring Plans | Market Intelligence
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			8
Ending Reserve Balance	2		2
Operating Segments | 2017 Restructuring Plans | Platts
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			1
Ending Reserve Balance	0		0
Operating Segments | 2017 Restructuring Plans | Indices
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			0
Ending Reserve Balance	0		0
Corporate
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded	7	$ 4	7	$ 4
Corporate | 2018 Restructuring Plans
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			7
Ending Reserve Balance	7		7
Corporate | 2017 Restructuring Plans
Restructuring Cost and Reserve [Line Items]
Initial Charge Recorded			10
Ending Reserve Balance	$ 3		$ 3