Significant Business Transactions - Acquisition of IFDS U.K., Schedule of Aggregate Acquisition-date Fair Value of Consideration Transferred (Details) - USD ($) $ in Millions			1 Months Ended
		Mar. 27, 2017	Mar. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill				$ 796.5	$ 516.4
International Financial Data Services U.K.
Consideration
Cash paid to acquire the remaining equity interests in IFDS U.K. and other related interests (1)		$ 141.0
Estimated fair value of previously-held equity interests		136.8
Recognized amounts of identifiable assets acquired and liabilities assumed
Cash and cash equivalents		99.2
Accounts receivable		101.7
Other current assets		14.4
Properties	[1]	95.6
Intangible assets		104.0
Goodwill		195.9
Deferred income taxes		11.5
Other assets		2.1
Total assets		624.4
Current portion of long-term debt		2.8
Accounts payable		29.1
Accrued compensation and benefits		23.6
Deferred revenue		31.1
Other current liabilities		61.7
Long-term debt		26.3
Other liabilities		23.6
Total liabilities		198.2
Net assets acquired		$ 426.2
Acquisition price discount (as a percent)		3.00%
IFDS U.K. IFDS, Percana and IFDS Realty U.K.
Consideration
Cash paid to acquire the remaining equity interests in IFDS U.K. and other related interests (1)	[2]	$ 234.9
Estimated fair value of previously-held equity interests	[3]	136.8
Effective settlement of pre-existing relationships		54.5
Total consideration transferred		426.2
Acquired software | International Financial Data Services U.K.
Recognized amounts of identifiable assets acquired and liabilities assumed
Properties		$ 21.9
Weighted average useful life			6 years

[1] Includes $21.9 million of acquired software with a weighted-average useful life of 6 years.

[2] Cash paid is comprised of cash payments to acquire State Street’s equity interest in IFDS U.K. and a note receivable from IFDS U.K., as well as IFDS L.P.’s equity interests in IFDS Percana and IFDS Realty U.K.

[3] Equals the estimated fair value of DST’s previously-held equity interest in IFDS U.K. valued at $136.8 million, which represents an approximate 3.0% discount to the acquisition price for State Street’s equity interests in IFDS U.K. prior to the acquisition date. The difference between the $141.0 million of cash paid to acquire State Street’s equity interests in IFDS U.K. and the $136.8 million represents an estimate of a control premium, which has not been included in the valuation of DST’s previous non-controlling interest.