Quarterly report pursuant to Section 13 or 15(d)

Related Party Transactions (Details)

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Related Party Transactions (Details) - NSP China - Subsidiaries - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Related Party Transaction [Line Items]      
Notes receivable, related parties, current $ 4,100,000 $ 4,100,000 $ 2,000,000
Company's Joint Venture Partner      
Related Party Transaction [Line Items]      
Notes receivable, related parties, current 1,000,000 1,000,000 $ 500,000
Notes Receivable      
Related Party Transaction [Line Items]      
Related party transaction, amount 0 $ 2,000,000  
Related party transaction, rate   3.00%  
Notes Receivable | Company's Joint Venture Partner      
Related Party Transaction [Line Items]      
Related party transaction, amount $ 0 $ 500,000