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Stock Plans - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value |  | 0 | 0 |
Stock options expiration period |  |  |  | 10 years
Stock options exercisable period after grant date |  | 3 years |  |
Restricted stock and restricted stock unit vest period after grant date |  | 3 years |  |
Average closing share price, period |  |  |  | 1 year
Cash proceeds from exercise of options |  | $ 8,573 | $ 18,465 | $ 12,476
Aggregate intrinsic value of stock options exercisable |  | 60,200 |  |
Aggregate intrinsic value of stock options outstanding |  | 69,300 |  |
Total intrinsic value of stock options exercised |  | 12,200 | 29,300 | 6,400
Unrecognized stock-based compensation expense |  | $ 8,100 |  |
Weighted average period over which unrecognized stock-based compensation expense is expected to be recognized, in years |  | 1 year 7 months 28 days |  |
Minimum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Percentage of target award based on achieving targeted performance conditions |  | 0.00% |  |
Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Percentage of target award based on achieving targeted performance conditions |  | 200.00% |  |
Percentage of target award based on achieving targeted return on net assets based on which final number of shares to be issued for performance stock units |  | 93.60% |  |
Stock Options |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Excess tax benefit (provision) related to stock plans |  | $ 2,500 | 7,400 | 600
Restricted Stock and Unit Activity |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Fair value of stock units vested |  | 5,400 | 4,100 | 3,700
Performance Stock Unit Activity |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Award vesting period (in years) | 3 years |  |  |
Fair value of stock units vested |  | $ 1,700 | $ 4,100 | $ 1,800
Management |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Shares reserved for future grants |  | 1,054,730 |  |
Non Employee Directors |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Shares reserved for future grants |  | 114,878 |  |
Stock Options |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock options expiration period |  | 10 years |  |

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