Significant Business Transactions and Events - Acquisition of BFDS, Schedule of Aggregate Acquisition-date Fair Value of Consideration Transferred (Details) - USD ($) $ in Millions		12 Months Ended
	Mar. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Goodwill		$ 799.1	$ 516.4	$ 458.3
Treasury stock acquired		$ 314.3	$ 315.8	$ 405.3
Boston Financial Data Services, Inc.
Consideration
Fair value of common stock used to acquire the remaining equity interests in BFDS, certain investments and real estate	$ 163.4
Estimated fair value of DST's previously-held equity interests	151.1
Effective settlement of pre-existing relationships	(5.9)
Total consideration transferred	308.6
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Cash and cash equivalents	96.8
Accounts receivable	81.6
Other current assets	3.6
Investments	35.8
Properties	22.6
Intangible assets	57.2
Goodwill	68.7
Deferred income taxes	2.2
Other assets	3.2
Total assets	371.7
Accounts payable	5.2
Accrued compensation and benefits	15.4
Deferred revenue	2.1
Other current liabilities	7.4
Other liabilities	33.0
Total liabilities	63.1
Net assets acquired	$ 308.6
Acquisition price discount (as a percent)	7.50%
Treasury stock acquired	$ 3.7
Acquired software | Boston Financial Data Services, Inc.
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Properties	$ 2.0
Weighted average useful life	5 years