Pension Plan - Additional Information (Detail) - USD ($) $ in Thousands |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Minimum required pension plan contribution | $ 5,100 | |
| Expected voluntary contribution for the year 2018 | 5,000 | |
| Available credit balance | 56,900 | |
| Net periodic pension income | 368 | $ 634 |
| Expected return on plan assets | 2,204 | 1,886 |
| Interest cost on projected benefit obligation | 1,114 | $ 1,084 |
| Pentegra Defined Benefit Plan [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Spinning off of asset cost | 2,800 | |
| Net periodic pension income | 62 | |
| Expected return on plan assets | 200 | |
| Recognized net actuarial gain | 3 | |
| Interest cost on projected benefit obligation | $ 100 | |