SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2014 USD ($)	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) component $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)	Mar. 31, 2013 USD ($)	Jun. 30, 2011 USD ($)
Significant Accounting Policies [Line Items]
Restricted cash		$ 700		$ 700		$ 400
Unbilled revenue within accounts receivable		8,100		$ 8,100		5,100
Period in which revenue is billed				30 days
Provision (benefit) for doubtful accounts		200	$ 100	$ 447	$ 362
Write-offs against the allowance for doubtful accounts		100	100	300	300
Property and equipment, accumulated depreciation		193,200		193,200		177,800
Software development cost capitalized		5,500	5,400	$ 11,600	9,900
Software development cost, amortization period (in years)				5 years
Capitalized software depreciation expense		4,100	3,600	$ 7,800	7,200
Carrying value of cost method investments		1,000		$ 1,000		1,100
Number of primary components associated with collaboration solutions | component				5
USF charges		$ 6,400	6,500	$ 12,700	$ 13,200
Number of shares repurchased | shares				1,529,354	1,408,559
Stock repurchase program, value of shares repurchased				$ 14,200	$ 18,200
Average price per share of stock repurchased | $ / shares				$ 9.29	$ 12.89
Shares withheld in satisfaction of employee tax withholding obligations | shares				138,652	89,651
Amount remitted related to tax withholding for share-based compensation				$ 1,200	$ 800
Preferred stock, shares authorized | shares		5,000,000		5,000,000
Preferred stock, par or stated value per share | $ / shares		$ 0.01		$ 0.01
Acquisition-related costs		$ 3,027	1,786	$ 3,600	3,691
State excise and sales tax reserve		10,100		10,100		9,900
Income tax expense		994	2,309	1,555	5,606
Unrecognized tax benefits		8,200		8,200		8,200
Unrecognized tax benefits that would affect effective tax rate, if recognized		6,800		6,800		6,900
Integration-related costs
Significant Accounting Policies [Line Items]
Acquisition-related costs		1,625	$ 1,036	2,560	$ 1,978
Integration-related costs | United Kingdom
Significant Accounting Policies [Line Items]
Acquisition-related costs				800
Investment in Cloud Solutions Provider
Significant Accounting Policies [Line Items]
Investment amount						1,000	$ 1,000
Investment in Conferencing Company
Significant Accounting Policies [Line Items]
Investment accounted for under cost method investment	$ 1,000
Investment amount								$ 1,000
Percent of investment sold	50.00%
Gain on sale of investment	$ 500
Market value of available-for-sale investment		$ 400		$ 400		$ 300
Minimum
Significant Accounting Policies [Line Items]
Contract term				12 months
Maximum
Significant Accounting Policies [Line Items]
Contract term				24 months
Furniture and Fixtures | Minimum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				5 years
Furniture and Fixtures | Maximum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				7 years
Software | Minimum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				2 years
Software | Maximum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				5 years
Computer, Communication and Network Equipment | Minimum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				3 years
Computer, Communication and Network Equipment | Maximum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)				5 years