SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) component $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)	Mar. 31, 2013 USD ($)	Jun. 30, 2011 USD ($)
Significant Accounting Policies [Line Items]
Restricted cash		$ 700,000		$ 700,000		$ 400,000
Unbilled revenue within accounts receivable		8,100,000		$ 8,100,000		5,100,000
Period in which revenue is billed				30 days
Provision for doubtful accounts		0	$ (200,000)	$ 467,000	$ 203,000
Write-offs against the allowance for doubtful accounts		0	100,000	300,000	500,000
Property and equipment, accumulated depreciation		201,300,000		201,300,000		177,800,000
Software development cost capitalized		6,900,000	6,000,000	$ 18,400,000	15,900,000
Software development cost, amortization period (in years)				5 years
Capitalized software depreciation expense		4,300,000	3,700,000	$ 12,100,000	10,900,000
Goodwill impairment						0
Carrying value of cost method investments		1,000,000		$ 1,000,000		1,100,000
Number of primary components associated with collaboration solutions | component				5
USF charges		$ 5,900,000	5,900,000	$ 18,600,000	$ 19,200,000
Number of shares repurchased | shares				1,529,354	1,901,498
Stock repurchase program, value of shares repurchased				$ 14,200,000	$ 24,600,000
Average price per share of stock repurchased | $ / shares				$ 9.29	$ 12.93
Shares withheld in satisfaction of employee tax withholding obligations | shares				182,218	112,266
Amount remitted related to tax withholding for share-based compensation				$ 1,400,000	$ 1,200,000
Preferred stock, shares authorized | shares		5,000,000		5,000,000
Preferred stock, par or stated value per share | $ / shares		$ 0.01		$ 0.01
Preferred stock, shares issued | shares		0		0
Preferred stock, shares outstanding | shares		0		0
Acquisition/divestiture-related costs		$ 2,421,000	2,147,000	$ 6,021,000	5,838,000
State excise and sales tax reserve		10,700,000		10,700,000		9,900,000
Income tax (benefit) expense		(553,000)	$ (376,000)	1,002,000	$ 5,230,000
Unrecognized tax benefits		7,400,000		7,400,000		8,200,000
Unrecognized tax benefits that would affect effective tax rate, if recognized		5,900,000		5,900,000		6,900,000
Change in valuation allowance				100,000
Integration-related costs | United Kingdom
Significant Accounting Policies [Line Items]
Acquisition/divestiture-related costs				800,000
Investment in Cloud Solutions Provider
Significant Accounting Policies [Line Items]
Investment amount						1,000,000	$ 1,000,000
Investment in Conferencing Company
Significant Accounting Policies [Line Items]
Investment amount								$ 1,000,000
Percent of investment sold	50.00%
Investment accounted for under cost method investment	$ 1,000,000
Gain on sale of investment	$ 500,000
Market value of available-for-sale investment		$ 1,100,000		$ 1,100,000		$ 300,000
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