Cash and Cash Equivalents, Short-term Investments and Restricted Cash - Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Market Value of Cash and Cash Equivalents and Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	$ 1,904,785	$ 1,141,263
Short-term investments:
Gross Unrealized Gains	617	526
Gross Unrealized Losses	(20)	(120)
Restricted cash	44,103	89,646
Cash, cash equivalents, short-term investments, and restricted cash, amortized cost	3,833,676	2,511,492
Cash, cash equivalents, short-term investments, and restricted cash, estimated fair market value	3,834,273	2,511,898
U.S. government agency securities
Short-term investments:
Amortized Cost	499,494	861,862
Gross Unrealized Gains	514	365
Gross Unrealized Losses	(17)	(73)
Estimated Fair Market Value	499,991	862,154
Corporate notes and bonds
Short-term investments:
Amortized Cost	15,611	159,382
Gross Unrealized Gains	24	91
Gross Unrealized Losses	0	(42)
Estimated Fair Market Value	15,635	159,431
Commercial paper
Short-term investments:
Amortized Cost		150,267
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Market Value		150,267
Treasury bills
Short-term investments:
Amortized Cost	1,360,011	79,989
Gross Unrealized Gains	76	14
Gross Unrealized Losses	(3)	0
Estimated Fair Market Value	1,360,084	80,003
Municipal securities
Short-term investments:
Amortized Cost	8,676	27,836
Gross Unrealized Gains	3	56
Gross Unrealized Losses	0	(3)
Estimated Fair Market Value	8,679	27,889
Certificates of deposit
Short-term investments:
Amortized Cost	996	1,245
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	996	1,245
Cash
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	150,556	194,955
Money market funds
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	1,747,818	872,431
U.S. government agency securities
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents, at cost		35,011
Cash and cash equivalents, gross unrealized losses		(2)
Cash and cash equivalents	1,162	35,009
Commercial paper
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents		31,113
Treasury bills
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	5,000	6,441
Corporate notes and bonds
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents		1,065
Certificates of deposit
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	$ 249	$ 249