Bank Loan Facilities - Additional Information (Details)				3 Months Ended	9 Months Ended
	Apr. 30, 2018 USD ($)	Oct. 12, 2017 USD ($)	Apr. 30, 2015 USD ($) Loan installment	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Oct. 10, 2017 USD ($)	Sep. 25, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Term loan facility				$ 0	$ 0				$ 16,875,000
Repayment of bank term loans					$ 16,875,000	$ 16,875,000
Cogent 12 Month Facility
Line of Credit Facility [Line Items]
Interest rate, percent			4.00%
Weighted average interest rate (percent)				5.17%	5.17%	4.67%
Cogent 36 Month Facility
Line of Credit Facility [Line Items]
Interest rate, percent			4.00%
Weighted average interest rate (percent)			1.25%
Term loan facility									$ 16,900,000
Debt instrument term (years)			3 years
Number of semi-annual installments (installment) | installment			4
Debt instrument, amount outstanding to reduce interest rate			$ 7,500,000
Repayment of bank term loans				$ 11,300,000
Cogent 36 Month Facility | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)			0.75%
Cogent Partners, LP
Line of Credit Facility [Line Items]
Term loan facility			$ 45,000,000
Number of bank term loan facilities (loan) | Loan			2
Cogent Partners, LP | Cogent 12 Month Facility | Short-term Debt
Line of Credit Facility [Line Items]
Term loan facility			$ 22,500,000
Cogent Partners, LP | Cogent 36 Month Facility | Long-term Debt
Line of Credit Facility [Line Items]
Term loan facility			$ 22,500,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving loan facility, maximum borrowing capacity				$ 90,000,000	$ 90,000,000
Increase in maximum borrowing capacity					$ 20,000,000
Interest rate, percent				3.50%	3.50%
Weighted average daily borrowings outstanding under the loan facility					$ 64,800,000	$ 52,200,000
Weighted average interest rate (percent)				4.04%	4.04%	3.50%
Revolving Credit Facility | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)					4.25%
Revolving Credit Facility | Recapitalization Credit Agreement | Secured Debt | Subsequent Event
Line of Credit Facility [Line Items]
Revolving loan facility, maximum borrowing capacity		$ 20,000,000
Debt instrument term (years)		3 years
Term Loan | Recapitalization Credit Agreement | Secured Debt
Line of Credit Facility [Line Items]
Revolving loan facility, maximum borrowing capacity								$ 300,000,000
Term Loan | Recapitalization Credit Agreement | Secured Debt | Subsequent Event
Line of Credit Facility [Line Items]
Revolving loan facility, maximum borrowing capacity		$ 350,000,000
Debt instrument term (years)		5 years
Authorized Repurchase Program on September 25 [Member] | Common Stock
Line of Credit Facility [Line Items]
Stock repurchase program, authorized amount								$ 235,000,000
Authorized Repurchase Program on September 25 [Member] | Common Stock | Subsequent Event
Line of Credit Facility [Line Items]
Stock repurchase program, authorized amount							$ 285,000,000
Stock Repurchase Program On October 10 | Common Stock | Subsequent Event
Line of Credit Facility [Line Items]
Stock repurchase program, authorized amount							$ 285,000,000
Scenario, Forecast | Cogent 36 Month Facility
Line of Credit Facility [Line Items]
Repayment of bank term loans	$ 5,600,000