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Investments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Investments Schedule [Abstract]        
Realized gains on trading securities $ 1.7 $ 0.2 $ 6.2 $ 0.5