Significant Business Transactions - Acquisitions of BFDS, Schedule of Aggregate Acquisition-date Fair Value of Consideration Transferred (Details) - USD ($) $ in Millions			9 Months Ended
		Mar. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill			$ 796.5		$ 516.4
Treasury stock acquired			$ 239.0	$ 240.7
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	[1]	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	[2]	35.8
Properties	[3]	22.6
Intangible assets		57.2
Goodwill		68.7
Deferred income taxes		2.4
Other assets		3.2
Total assets		371.9
Accounts payable		5.2
Accrued compensation and benefits		15.4
Deferred revenue		2.1
Other current liabilities		7.6
Other liabilities		33.0
Total liabilities		63.3
Net assets acquired		$ 308.6
Acquisition price discount (as a percent)		7.50%
Treasury stock acquired		$ 3.7
Boston Financial Data Services, Inc. | Acquired software
Recognized amounts of identifiable assets acquired and liabilities assumed
Properties		$ 2.0
Weighted average useful life		5 years

[1] Equals the estimated fair value of DST’s previously-held equity interest in BFDS valued at $151.1 million, which represents an approximate 7.5% discount to the acquisition price for State Street’s equity interests in BFDS prior to the acquisition date. The difference between the fair value of State Street common stock transferred of $163.4 million and the $151.1 million represents an estimate of a control premium, which has not been included in the valuation of DST’s previous non-controlling interest.

[2] As a result of the acquisition of the remaining interests in BFDS, we acquired certain investments associated with active deferred compensation plans for senior management and certain highly compensated employees. Approximately $3.7 million of the underlying investments were in DST common stock. As a result, the common stock was considered effectively repurchased at the acquisition date and reclassified to Treasury stock in the Condensed Consolidated Balance Sheet.

[3] Includes $2.0 million of acquired software with a weighted-average useful life of 5 years.