Fair Value Measurements - Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Less than one year, amortized cost	$ 859,996	$ 261,468
One to three years, amortized cost	734,338	0
Total amortized cost	1,594,334	261,468
Less than one year, aggregate fair value	861,181	261,128
One to three years, aggregate fair value	737,852	0
Total aggregate fair value	$ 1,599,033	$ 261,128