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Accounts Receivable Accounts Receivable (Tables)
 12 Months Ended
Dec. 31, 2017
Accounts Receivable, Net [Abstract]  
Schedule of Accounts, Notes, Loans and Financing Receivable
Accounts receivable consisted of the following at December 31, 2017 and 2016 (dollars in thousands): 
 
 
2017
 
2016
Accounts receivable - trade
 
$
401,723

 
$
353,347

Accounts receivable - grants from outside parties
 
17,734

 
10,652

Accounts receivable - insurance and other third-party claims
 
10,753

 
11,994

Total accounts receivable
 
430,210

 
375,993

Allowance for doubtful accounts
 
(13,505
)
 
(12,070
)
Accounts receivable, net
 
$
416,705

 
$
363,923
Schedule of Allowance for Doubtful Accounts
Activity in the Company's allowance for doubtful accounts for the years ended December 31, 2017, 2016 and 2015 was as follows (dollars in thousands): 
 
 
2017
 
2016
 
2015
Balance, beginning of year
 
$
12,070

 
$
6,213

 
$
5,826

Provisions
 
7,752

 
19,655

 
7,512

Charges
 
(6,317
)
 
(13,798
)
 
(7,125
)
Balance, end of year
 
$
13,505

 
$
12,070

 
$
6,213