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Note Debt Securities Available for Sale (Debt securities available- for-sale - Additional Information) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]          
Other Than Temporary Impairment Losses Investments Portion Recognized In Earnings Net $ 0 $ 8,299 $ 0 $ 8,299  
Proceeds From Sale Of Available For Sale Securities Debt     0 0  
Gross Unrealized Losses $ 280,083   280,083   $ 133,085
Debt Securities, Available-for-sale, Realized Gain (Loss)     $ 0 $ 0