Segment and Geographic Information - Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment information
Revenue	$ 1,589	$ 1,513	$ 1,509	$ 1,453	$ 1,399	$ 1,439	$ 1,482	$ 1,341	$ 6,063	$ 5,661	$ 5,313
Operating Profit	628	$ 658	$ 677	$ 648	857	1,348	$ 651	$ 512	2,610	3,369	1,917
Amortization of intangibles									98	96	67
Pre-tax gain (loss) on dispositions	(379)					$ (722)			(1,100)
Depreciation & Amortization									180	181	157
Capital Expenditures									123	115	139
Total Assets	9,425				8,669				9,425	8,669
Assets of a business held for sale	5				7				5	7
Operating segments
Segment information
Revenue									6,063	5,661	5,313
Operating Profit									2,788	3,496	2,055
Depreciation & Amortization									171	173	150
Capital Expenditures									100	102	130
Total Assets	6,230				5,963				6,230	5,963
Intersegment elimination
Segment information
Revenue									(110)	(98)	(88)
Segment reconciling items
Segment information
Operating Profit									(178)	(127)	(138)
Restructuring costs									10		10
Lease exit costs									19
Pre-tax gain (loss) on dispositions										(3)	(11)
Pension settlement charge									8
Corporate
Segment information
Depreciation & Amortization									9	8	7
Capital Expenditures									23	13	9
Total Assets	3,190				2,699				3,190	2,699
Ratings
Segment information
Restructuring costs									25	6	13
Insurance recoveries										10	54
Amortization of intangibles									4	5	5
Total Assets	788				612				788	612
Ratings | Operating segments
Segment information
Revenue									2,988	2,535	2,428
Operating Profit									1,524	1,262	1,078
Legal fees									55
Depreciation & Amortization									34	34	43
Capital Expenditures									45	42	48
Market and Commodities Intelligence
Segment information
Restructuring costs											33
Amortization of intangibles									87	85	57
Non-cash acquisition and disposition-related adjustments									15	48
Lease exit costs									6
Asset write-off									2
Pre-tax gain (loss) on dispositions										(1,100)
Impairment of intangible assets										24
Acquisition-related costs										1	37
Total Assets	4,172				4,104				4,172	4,104
Market and Commodities Intelligence | Operating segments
Segment information
Revenue									2,452	2,585	2,376
Operating Profit									793	1,822	585
Restructuring costs									9
Depreciation & Amortization									128	131	99
Capital Expenditures									52	57	78
Indices
Segment information
Amortization of intangibles									7	6	5
Total Assets	$ 1,270				$ 1,247				1,270	1,247
Indices | Operating segments
Segment information
Revenue									733	639	597
Operating Profit									471	412	392
Depreciation & Amortization									9	8	8
Capital Expenditures									$ 3	$ 3	$ 4