Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 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