|
Financial Highlights (Per Share Performance Data and Other Supplemental Financial Data) (Detail) (USD $)
|
12 Months Ended | |||
|---|---|---|---|---|
|
Dec. 31, 2013
|
Dec. 31, 2012
|
Dec. 31, 2011
|
Feb. 26, 2008
|
|
| Per Share Operating Performance: | ||||
| Net asset value, beginning of year | $ 58.39 | $ 48.37 | $ 42.06 | $ 50.00 |
| Total income | $ 2.05 | $ 10.50 | $ 6.72 | |
| Net expenses | $ (0.55) | $ (0.48) | $ (0.41) | |
| Net increase in net asset value | $ 1.50 | $ 10.02 | $ 6.31 | |
| Net asset value, end of year | $ 59.89 | $ 58.39 | $ 48.37 | $ 50.00 |
| Total Return | 2.57% | 20.72% | 15.00% | |
| Ratios to Average Net Assets | ||||
| Total income | 5.99% | 16.34% | 13.24% | |
| Management fees | 0.60% | 0.60% | 0.60% | |
| Total expenses excluding management fees | 0.57% | 0.43% | 0.33% | |
| Expenses waived | (0.24%) | (0.15%) | (0.09%) | |
| Net expenses excluding management fees | 0.33% | 0.28% | 0.24% | |
| Net income | 5.06% | 15.46% | 12.40% | |