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Investment Securities - Additional Information (Detail)
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
Investment
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]      
Gross realized gains on sale of investment securities $ 0 $ 0  
Number of investment securities with aggregate gross unrealized losses | Investment 1,038    
Unrealized losses on individual investment securities $ 146,074,000   $ 186,233,000
Cost method investment securities $ 499,000,000