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Summary Of Significant Accounting Policies - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
MA [Member] | Subscription and Maintenance Contracts [Member] | Advance Payment [Member]  
Capitalized Contract Cost [Line Items]  
Contractual Coverage Period 1 year
MA [Member] | Minimum [Member]  
Capitalized Contract Cost [Line Items]  
Payment Terms 30 days
MA [Member] | Minimum [Member] | Subscription and Maintenance Contracts [Member]  
Capitalized Contract Cost [Line Items]  
Contractual Coverage Period 3 years
MA [Member] | Maximum [Member]  
Capitalized Contract Cost [Line Items]  
Payment Terms 60 days
MA [Member] | Maximum [Member] | Subscription and Maintenance Contracts [Member]  
Capitalized Contract Cost [Line Items]  
Contractual Coverage Period 5 years
MA [Member] | SG&A Expense [Member]  
Capitalized Contract Cost [Line Items]  
Deferred Cost Balance $ 95.5
Contract Cost Amortization 8.6
MA [Member] | Royalty Cost [Member] | Operating Expense [Member]  
Capitalized Contract Cost [Line Items]  
Deferred Cost Balance 32.4
Contract Cost Amortization $ 14.2
MIS [Member] | Monitoring of Issuers or Issued Securities [Member]  
Capitalized Contract Cost [Line Items]  
Contractual Coverage Period 1 year
MIS [Member] | Capitalization of Work-In-Process for In-Progress Ratings [Member] | Operating Expense [Member]  
Capitalized Contract Cost [Line Items]  
Deferred Cost Balance $ 10.2
Contract Cost Amortization $ 9.4