Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 788,200	$ 979,000
Short-term investment securities	106,200	105,300
Assets held for sale	2,300	2,300
Total assets	896,700	1,086,600
Total liabilities	0	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	788,200	979,000
Short-term investment securities	106,200	105,300
Assets held for sale	0
Total assets	894,400	1,084,300
Total liabilities	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Short-term investment securities	0	0
Assets held for sale	0
Total assets	0	0
Total liabilities	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Short-term investment securities	0	0
Assets held for sale	2,300	2,300
Total assets	2,300	2,300
Total liabilities	$ 0	$ 0