Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	$ 1,225,970	$ 840,890
Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	1,219,921	720,086
Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	6,049	120,804
Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	0	0
Investment securities
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	6,049	114,278
Long-term		6,526
Investment securities | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	0	0
Long-term		0
Investment securities | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	6,049	114,278
Long-term		6,526
Investment securities | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	0	0
Long-term		0
Certificate of deposit
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	5,076	5,000
Certificate of deposit | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	5,076	5,000
Certificate of deposit | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	0	0
Certificate of deposit | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	0	0
Money market funds and other funds
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	1,214,845	715,086
Money market funds and other funds | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	1,214,845	715,086
Money market funds and other funds | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	0	0
Money market funds and other funds | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	$ 0	$ 0