Introduction and Basis of Presentation - Additional Information (Detail) - Operating Revenues [Member] - Customer Concentration Risk [Member] - Customer	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Introduction And Basis Of Presentation [Line Items]
Number of major customers represented 10.0% or more of revenues		0
BlackRock Inc [Member]
Introduction And Basis Of Presentation [Line Items]
Percentage of operating revenues accounted for by major customer	10.30%
Index [Member] | BlackRock Inc [Member]
Introduction And Basis Of Presentation [Line Items]
Percentage of operating revenues accounted for by major customer	18.60%	17.00%
Analytics [Member]
Introduction And Basis Of Presentation [Line Items]
Number of major customers represented 10.0% or more of revenues	0	0
All Other [Member]
Introduction And Basis Of Presentation [Line Items]
Number of major customers represented 10.0% or more of revenues	0	0