Significant Business Transactions and Events - Summary of BFDS and IFDS U.K. Pro Forma Results (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
BFDS and IFDS U.K.
Business Acquisition [Line Items]
Total revenue	$ 2,366.3	$ 2,327.0
Net income attributable to DST Systems, Inc.	$ 273.0	$ 386.3
Diluted earnings per share	$ 4.40	$ 5.79
International Financial Data Services U.K.
Business Acquisition [Line Items]
Total revenue of acquiree since acquisition date	$ 436.7
Net pretax income or loss acquiree since acquisition date	79.0
Boston Financial Data Services, Inc.
Business Acquisition [Line Items]
Total revenue of acquiree since acquisition date	206.9
Net pretax income or loss acquiree since acquisition date	$ 28.8