Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity Common Stock Rollforward (Details)

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Stockholders' Equity Common Stock Rollforward (Details)
 6 Months Ended 12 Months Ended
Nov. 01, 2017
shares
Jun. 30, 2018
shares
Jun. 30, 2017
shares
Dec. 31, 2017
shares
Oct. 31, 2017
shares
Dec. 31, 2016
shares
Increase (Decrease) in Common Stock Outstanding [Roll Forward]            
Common shares outstanding at beginning of period (in shares)   174,496,587        
Common shares outstanding at end of period (in shares)   175,470,398   174,496,587    
Reverse stock split, conversion ratio 0.50          
Common shares authorized (in shares) 450,000,000 450,000,000   450,000,000 900,000,000  
Number of nonvested restricted common shares outstanding (in shares)   1,593,701 1,329,163 1,284,010   1,319,712
Common Stock [Member]            
Increase (Decrease) in Common Stock Outstanding [Roll Forward]            
Common shares outstanding at beginning of period (in shares)   174,496,587 173,826,163 173,826,163    
Number of shares of stock issued during period (in shares)   12,468 13,219      
Number of shares of restricted common stock issued during period (in shares)   961,343 649,225      
Common shares outstanding at end of period (in shares)   175,470,398 174,488,607 174,496,587