Commitments and Contingencies - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended			1 Months Ended
	Apr. 30, 2008	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Sep. 30, 2012	May 31, 2012	Oct. 31, 2010	Dec. 31, 2011	Mar. 31, 2014	Apr. 30, 2014	Jul. 31, 2014	Nov. 30, 2009	Oct. 31, 2014	Jun. 30, 2011	Aug. 31, 2013
Commitments And Contingencies [Line Items]
Amended contract year	3 years
Contractual obligations includes both fixed and variable				$ 100,000			$ 100,000
Rent expense				18,600,000	19,100,000	20,400,000
Lease incentives				426,000	180,000		426,000
Deposits related to letter of credit are included in other assets				100,000
Reduced LOC		100,000	200,000
Operating Leases, Future Minimum Payments Due				22,514,000			22,514,000
Minimum [Member]
Commitments And Contingencies [Line Items]
LOC contract period requirement				1 year
Operating leases, term of contract				1 month
Maximum [Member]
Commitments And Contingencies [Line Items]
LOC contract period requirement				3 years
Operating leases, term of contract				5 years
Telecommunication services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				200,000			200,000
Long-term Purchase Commitment, Period							3 years	2 years	2 years
Long-term Purchase Commitment, Amount							200,000
Computer support services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				100,000			100,000
Long-term Purchase Commitment, Period										3 years
Satellite services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				300,000			300,000
Long-term Purchase Commitment, Period											3 years
Network Maintenance Services [Member]
Commitments And Contingencies [Line Items]
Long-term Purchase Commitment, Period												3 years
Long-term Purchase Commitment, Adjustment Notice Period												90 days
Remaining contractual obligations				100,000			100,000
Paging Equipment Assets [Member]
Commitments And Contingencies [Line Items]
Long-term Purchase Commitment, Period													3 years
Long-term Purchase Commitment, Amount													8,300,000
Internet and Telephone Services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				600,000			600,000
Sales and Customer Support Technology Services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				1,300,000			1,300,000
Long-term Purchase Commitment, Period														3 years
Long-term Purchase Commitment, Amount														1,600,000
Information Technology Services [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				100,000			100,000
Long-term Purchase Commitment, Period															33 months
Long-term Purchase Commitment, Amount															100,000
VIRGINIA
Commitments And Contingencies [Line Items]
Operating Leases, Future Minimum Payments Due														1,200,000
Headquarter Office Space Lease [Member] | VIRGINIA
Commitments And Contingencies [Line Items]
Lease commencement year				April 2010
Rent expense				1,400,000
Lease incentives				400,000			400,000
Duration of lease term																5 years
Remaining commitment				1,200,000			1,200,000
Operating leases, term of contract														3 years
Lease Expiration Date														Mar. 31, 2018
Office Space Adjacent to Headquarters [Member] | VIRGINIA
Commitments And Contingencies [Line Items]
Operating leases, term of contract														4 years
Lease Expiration Date														Mar. 31, 2018
Office space lease [Member] | Texas [Member]
Commitments And Contingencies [Line Items]
Lease commencement year				October 2011
Rent expense				2,500,000
Duration of lease term																		5 years
Remaining commitment				2,200,000			2,200,000
Rent abatements				200,000
Operating leases, term of contract																	2 years
Operating Leases, Future Minimum Payments Due																	1,100,000
Office space lease [Member] | Minnesota [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				1,300,000			1,300,000
Office space lease [Member] | New York [Member]
Commitments And Contingencies [Line Items]
Rent expense				3,600,000
Lease incentives				400,000			400,000
Duration of lease term																			10 years
Remaining commitment				3,500,000			3,500,000
Office space lease [Member] | New Hampshire [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				600,000			600,000
Office space lease [Member] | Florida [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				100,000			100,000
Office space lease [Member] | Australia [Member]
Commitments And Contingencies [Line Items]
Remaining commitment				$ 200,000			$ 200,000