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Fair Value Measurements - Additional Information (Detail) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Debt Instrument Fair Value Disclosure [Abstract]      
Transfer of securities between levels of fair value hierarchy $ 0 $ 0  
Days from origination after which mortgage loans are sold 30 days    
Amount of non-financial assets (liabilities) measured at fair value on a recurring basis $ 0   $ 0
Non-financial assets or liabilities for which no impairment was provided $ 0   $ 0