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Investments - Summary of Available-for-Sale Debt Securities and Equity Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Available-for-Sale Debt Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Proceeds from sales and maturities (fair value) $ 124,827 $ 19,829
Gross realized gains on sale of securities 307  
Gross realized losses on sale of securities (3,072) (36)
Equity Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Proceeds from sales and maturities (fair value) 1,045 2,500
Gross realized gains on sale of securities $ 124  
Gross realized losses on sale of securities   $ (27)