Available-for-Sale Debt Securities - Additional Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) position	Dec. 31, 2018 USD ($) position
Debt Securities, Available-for-sale [Abstract]
Securities in unrealized loss positions, number of positions (securities) | position	915	1,348
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale | $	$ 0	$ 0