Available-for-Sale Securities - Summary of Fair Value and Gross Unrealized Losses of Available-for-Sale Securities in a Continuous Unrealized Loss Position (Details) - U.S. Treasury Securities $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Fair Value	$ 256,278
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	$ (12)