Annual report pursuant to Section 13 and 15(d)

Summary of Potential Number of Units that Could Vest Under Restricted Stock Unit Awards (Detail)

v3.8.0.1
Summary of Potential Number of Units that Could Vest Under Restricted Stock Unit Awards (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Restricted Stock Unit [Abstract]      
Number of units at threshold IRR 58,545 84,554 47,640
Number of units at target IRR 117,089 169,107 95,282
Number of units at maximum IRR 175,634 253,661 142,917
Fair value of units at threshold IRR [1] $ 2,481 $ 2,522 $ 2,057
Fair value of units at target IRR [1] 4,961 5,044 4,115
Fair value of units at maximum IRR [1] $ 7,442 $ 7,568 $ 6,173
[1] The grant date fair values for units issued during the years ended December 31, 2015, 2016, and 2017 were $43.19, $29.83 and $42.37, respectively.