Summary of Significant Accounting Policies - Accounting for Stock-Based Compensation (Detail)	12 Months Ended
	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options grants in period (in shares)	0
2006 Equity Incentive Plan (The Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock authorized under 2006 Equity Incentive Plan (in shares)	597,557
Number of authorized common stock shares (in shares)	437,481
Maximum amount of shares a single participant can be awarded annually (in shares)	100,000
Contractual life of awards granted under 2006 Equity Incentive Plan	10 years
2013 ESPP Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of authorized common stock shares (in shares)	525,000
Percentage of the total number of shares of common stock outstanding of the preceding year	10.00%
Maximum amount of shares of common stock that employee can purchase per year (in shares)	100,000
Maximum percentage of annual compensation that employee can use to purchase common stock shares	15.00%
Percent of fair maket value that common stock can be purchased at	85.00%