Available-for-Sale Debt Securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Abstract]
Securities in unrealized loss positions, number of positions (securities) | security	1,348	2,957
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale | $	$ 0	$ 0