Significant Business Transactions and Events - Acquisition of kasina, Red Rocks and Wealth Management Systems (Details) - USD ($) $ in Millions				1 Months Ended		12 Months Ended
	Aug. 21, 2015	Jul. 31, 2015	Jan. 01, 2015	Jan. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Goodwill					$ 458.3	$ 458.3	$ 799.1	$ 516.4
kasina LLC
Business Acquisition [Line Items]
Performance-related contingent consideration			$ 2.1
Consideration
Cash paid			$ 9.0
Red Rock Capital LLC
Business Acquisition [Line Items]
Performance-related contingent consideration		$ 20.0
Remaining contingent consideration available							$ 1.0
Consideration
Cash paid		$ 45.0
Wealth Management Systems Inc.
Business Acquisition [Line Items]
Working capital adjustment				$ 0.2	$ 1.1
Consideration
Cash paid	$ 65.1
Series of Individually Immaterial Business Acquisitions
Business Acquisition [Line Items]
Goodwill, expected tax deductible amount	40.0
Consideration
Cash paid	118.8
Fair value of contingent consideration	0.8
Total consideration transferred	119.6
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Cash and cash equivalents	1.6
Accounts receivable	4.2
Other current assets	0.2
Properties	3.3
Intangible assets	49.5
Goodwill	75.8
Total assets	134.6
Accounts payable	0.2
Accrued compensation and benefits	0.4
Deferred revenue	5.5
Other current liabilities	0.7
Deferred income tax liabilities	8.2
Total liabilities	15.0
Net assets acquired	119.6
Finite-lived Intangible Assets Acquired	49.5
Total revenue of acquiree since acquisition date						10.4
Net pretax income or loss acquiree since acquisition date						$ (9.3)
Series of Individually Immaterial Business Acquisitions | Acquired software
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Properties	3.0
Series of Individually Immaterial Business Acquisitions | Client relationships
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Finite-lived Intangible Assets Acquired	$ 48.0
Weighted average useful life	17 years
Series of Individually Immaterial Business Acquisitions | Other intangible assets
Recognized Amounts of Identifiable Assets Acquired and Liabilities Assumed
Finite-lived Intangible Assets Acquired	$ 1.5
Weighted average useful life	7 years