Fair Value Measurements - Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Less than one year, amortized cost	$ 787,710	$ 859,996
One to three years, amortized cost	716,571	734,338
Total amortized cost	1,504,281	1,594,334
Less than one year, aggregate fair value	788,246	861,181
One to three years, aggregate fair value	709,623	737,852
Total aggregate fair value	$ 1,497,869	$ 1,599,033