Segment Information - Operating Revenue by Reportable Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Total	$ 288,433	$ 268,771	$ 857,857	$ 802,120
Index [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total	157,751	141,577	454,481	415,262
Analytics [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total	111,291	108,341	333,947	322,756
All Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total	$ 19,391	$ 18,853	$ 69,429	$ 64,102