Acquisitions, Goodwill, and Other Intangible Assets Acquisition, Goodwill, and Other Intangible Assets - Pro Forma (Details) - DBRS [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenue	$ 1,259.2	$ 1,184.5
Operating income	190.3	223.6
Net income	$ 148.2	$ 179.7
Basic net income per share attributable to Morningstar, Inc. (in dollars per share)	$ 3.47	$ 4.22
Diluted net income per share attributable to Morningstar, Inc. (in dollars per share)	$ 3.43	$ 4.18