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Summary of Significant Accounting Policies - Interest Cost Capitalized and Incurred (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]                      
Interest incurred                 $ 2,656 $ 2,538 $ 1,962
Interest capitalized                 487 1,210 714
Interest expense $ 527 $ 567 $ 495 $ 580 $ 497 $ 344 $ 158 $ 329 $ 2,169 $ 1,328 $ 1,248