Stock - Based Compensation - Summary of Accumulated Stock Units (Detail) - $ / shares |
Sep. 30, 2017 |
Dec. 31, 2016 |
---|---|---|
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] | ||
Accumulated stock units | 136,679 | 140,044 |
Average price | $ 22.33 | $ 20.87 |