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Schedule II Valuation and Qualifying Accounts (Tables)
 12 Months Ended
Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]  
Allowance for doubtful accounts and billing adjustments
Years Ended December 31, 2013, 2014, and 2015 (in thousands):

Allowance for Doubtful Accounts and Billing Adjustments (1)
 
Balance at
Beginning
of Year
 
Charged to
Expense
 
Charged to
Other
Accounts (2)
 
Write-offs,
Net of
Recoveries
 
Balance at
End of Year
Year ended December 31, 2013
 
$
2,935

 
$
2,317

 
$
—

 
$
1,855

 
$
3,397

Year ended December 31, 2014
 
$
3,397

 
$
4,822

 
$
881

 
$
4,285

 
$
4,815

Year ended December 31, 2015
 
$
4,815

 
$
7,002

 
$
1,470

 
$
5,809

 
$
7,478


(1) 
Additions to the allowance for doubtful accounts are charged to bad debt expense.

(2) 
Amounts represent opening balances from acquired businesses.