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Investments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale investment securities $ 393,576 $ 380,188
Number of securities with unrealized gross losses 209 89