Nature of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Percentage of receivables as of December 31, 2017, 2016		No customer accounted for more than 10% of the Company's consolidated receivables balance as of December 31, 2017 or 2016.
Amount of off balance sheet settlement funds		$ 238,900,000	$ 270,000,000
Goodwill measurement period		1 year
Goodwill		$ 909,691,000	909,691,000
Goodwill impairment		$ 0
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life		3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life		20 years
ACI On Premise [Member]
Summary Of Significant Accounting Policies [Line Items]
Goodwill		$ 725,900,000
ACI On Demand [Member]
Summary Of Significant Accounting Policies [Line Items]
Goodwill		183,800,000
Adjustments for New Accounting Pronouncement [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings, net of tax		$ 273,000,000
Scenario, Forecast [Member] | Adjustments for New Accounting Pronouncement [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to retained earnings, net of tax	$ 291,000,000
Software Acquired or Developed for Internal Use [Member] | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life		3 years
Software Acquired or Developed for Internal Use [Member] | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life		10 years
Level 2 [Member]
Summary Of Significant Accounting Policies [Line Items]
Fair value senior note		$ 305,700,000	$ 309,800,000