Equity-Based Compensation - Restricted Stock Awards with Performance Conditions (Details) - Restricted stock - $ / shares	3 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
ACTA Stock Price at Issuance (in dollars per share)	$ 13.66	$ 9.66	$ 15.30	$ 11.56	$ 13.33	$ 18.75
Restricted Shares Granted (in shares)		54,310	286,256		112,397	434,283
Restricted Shares Vested (in shares)	8,500	81,897	625,646	16,201	81,391	657,317
Restricted Shares Forfeited (in shares)			167,712		7,500	588,819
Actua 2015 Performance Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of Target Bonus Potentially Payable in Restricted Stock									150.00%
ACTA Stock Price at Issuance (in dollars per share)									$ 16.76
Restricted Shares Granted (in shares)									316,715
Performance Plan Achievement Percentage									83.00%
Restricted Shares Vested (in shares)									175,249
Restricted Shares Forfeited (in shares)									141,466
Actua 2016 Performance Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of Target Bonus Potentially Payable in Restricted Stock								100.00%
ACTA Stock Price at Issuance (in dollars per share)								$ 8.44
Restricted Shares Granted (in shares)								419,283
Performance Plan Achievement Percentage								60.00%
Restricted Shares Vested (in shares)								251,570
Restricted Shares Forfeited (in shares)								167,713
Actua 2017 Performance Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of Target Bonus Potentially Payable in Restricted Stock							100.00%
ACTA Stock Price at Issuance (in dollars per share)							$ 13.85
Restricted Shares Granted (in shares)							263,756