Available for Sale Securities - Summary of Available for Sale Securities that were in Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Unrealized Loss for Less Than 12 Months, Grosss Unrealized Losses	$ (8)	$ (223)
Unrealized Loss for Less Than 12 Months, Estimated Fair Value	25,741	174,456
Unrealized Loss for 12 Months or More, Gross Unrealized Losses	(50)	(39)
Unrealized Loss for 12 Months or More, Estimated Fair Value	39,793	29,934
Total, Gross Unrealized Losses	(58)	(262)
Total, Estimated Fair Value	65,534	204,390
U.S. government debt securities
Schedule Of Available For Sale Securities [Line Items]
Unrealized Loss for Less Than 12 Months, Grosss Unrealized Losses	(4)	(159)
Unrealized Loss for Less Than 12 Months, Estimated Fair Value	15,205	116,026
Unrealized Loss for 12 Months or More, Gross Unrealized Losses	(50)	(39)
Unrealized Loss for 12 Months or More, Estimated Fair Value	39,793	29,934
Total, Gross Unrealized Losses	(54)	(198)
Total, Estimated Fair Value	54,998	145,960
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Unrealized Loss for Less Than 12 Months, Grosss Unrealized Losses	(4)	(64)
Unrealized Loss for Less Than 12 Months, Estimated Fair Value	10,536	58,430
Total, Gross Unrealized Losses	(4)	(64)
Total, Estimated Fair Value	$ 10,536	$ 58,430