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ETF Shareholder Proxy Solicitation - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Stockholders Equity Note [Abstract]          
Percentage of common stock own by beneficial owner         25.50%
Reduced percentage of common stock owned by beneficial owner due to sell or transfer         25.00%
Threshold percentage of ownership presumes change in control         25.00%
ETF shareholders proxy    $ 3,198    $ 3,264 $ 3,264