Bank Loan Facilities - Additional Information (Detail)		1 Months Ended			12 Months Ended
	Apr. 01, 2015 USD ($) Loan installment	Apr. 30, 2016 installment	Jun. 30, 2015 installment	Apr. 30, 2015 installment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Bank term loans payable					$ 16,875,000	$ 33,750,000
Repayments of bank term loans					$ 16,875,000	$ 11,250,000	$ 0
Cogent 12 Month Facility
Line of Credit Facility [Line Items]
Debt interest rate	4.00%
Weighted average interest rate (percent)	0.75%				4.70%	4.30%
Debt term (years)	1 year
Number of equal payment installments | installment		1	1	2
Cogent 36 Month Facility
Line of Credit Facility [Line Items]
Debt interest rate	4.00%
Weighted average interest rate (percent)	1.25%				4.70%	4.30%
Bank term loans payable					$ 16,900,000
Debt term (years)	3 years
Number of semi-annual installments | installment	4
Amount outstanding to reduce interest rate	$ 7,500,000
Repayments of bank term loans					5,600,000
Cogent Partners, LP
Line of Credit Facility [Line Items]
Bank term loans payable	$ 45,000,000
Number of bank term loan facilities (loan) | Loan	2
Cogent Partners, LP | Long-term Debt | Cogent 36 Month Facility
Line of Credit Facility [Line Items]
Bank term loans payable	$ 22,500,000
Cogent Partners, LP | Short-term Debt | Cogent 12 Month Facility
Line of Credit Facility [Line Items]
Bank term loans payable	$ 22,500,000
Prime Rate | Cogent 36 Month Facility
Line of Credit Facility [Line Items]
Spread on interest rate (percent)	0.75%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Amount outstanding on letters of credit					$ 70,000,000	$ 50,000,000.0
Debt interest rate					3.50%
Weighted average daily borrowings outstanding under the loan facility					$ 54,100,000	$ 35,800,000
Weighted average interest rate (percent)					3.50%	3.30%	3.30%
Revolving Credit Facility | Prime Rate
Line of Credit Facility [Line Items]
Spread on interest rate (percent)					3.75%