Significant Business Transactions - Acquisitions of BFDS (Details) - USD ($) shares in Millions, $ in Millions		Mar. 30, 2017	Sep. 30, 2017	Mar. 29, 2017	Mar. 27, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Ownership percentage			50.00%			50.00%
Goodwill			$ 796.5			$ 516.4
Boston Financial Data Services, Inc.
Business Acquisition [Line Items]
Ownership percentage				50.00%
Equity issued (in shares)		2.0
Fair value of common stock used to acquire the remaining equity interests in BFDS, certain investments and real estate		$ 163.4
Cost basis for tax purposes		1.1
Negotiated fair value		157.6
Estimated fair value of DST’s previously-held equity interests	[1]	151.1
Remeasurement gain		56.0
Goodwill		68.7
Goodwill expected tax deductible amount		$ 0.0
International Financial Data Services L.P.
Business Acquisition [Line Items]
Ownership percentage		50.00%	50.00%		50.00%

[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.