Fair Value Measurements - Schedule of Reconciliation of Debt Securities (Detail) - Debt Securities [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] | ||
| Beginning balance | $ 16,050 | $ 12,220 |
| Accrued interest income | 0 | 0 |
| Gains or losses recorded for the period | ||
| Recognized in net income | 0 | (5,585) |
| Recognized in other comprehensive income | 0 | 2,365 |
| Ending balance | $ 16,050 | $ 9,000 |