XML 71 R55.htm IDEA: XBRL DOCUMENT v3.8.0.1
Related Party Transactions - Additional Information (Detail) - Elm Street
 12 Months Ended
Dec. 31, 2017
USD ($)
lot
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Related Party Transaction [Line Items]      
Related party forward lot purchase agreements purchase price $ 66,600,000    
Number of related parties for forward lot purchase agreement | lot 1    
Market price of developed lots $ 37,100,000 $ 44,500,000 $ 41,200,000
Expected number of lots from joint venture with Elm Street | lot 2,400    
Additional funding to joint venture by NVR and Elm Street $ 2,900,000    
Development costs to manage property under related party transactions $ 0 $ 143,000 $ 143,000