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	Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	$ 1,141,263	$ 651,058
Short-term investments:
Amortized Cost	1,280,581
Gross Unrealized Gains	526	76
Gross Unrealized Losses	(120)	(1,046)
Estimated Fair Market Value	1,280,989
Restricted cash	89,646	12,385
Cash, cash equivalents, short-term investments, and restricted cash, amortized cost	2,511,492	1,568,280
Cash, cash equivalents, short-term investments, and restricted cash, estimated fair market value	2,511,898	1,567,310
U.S. government agency securities
Short-term investments:
Amortized Cost	861,862	647,266
Gross Unrealized Gains	365	51
Gross Unrealized Losses	(73)	(821)
Estimated Fair Market Value	862,154	646,496
Corporate notes and bonds
Short-term investments:
Amortized Cost	159,382	113,109
Gross Unrealized Gains	91	1
Gross Unrealized Losses	(42)	(177)
Estimated Fair Market Value	159,431	112,933
Commercial paper
Short-term investments:
Amortized Cost	150,267	85,506
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	150,267	85,506
Treasury bills
Short-term investments:
Amortized Cost	79,989
Gross Unrealized Gains	14
Gross Unrealized Losses	0
Estimated Fair Market Value	80,003
Municipal securities
Short-term investments:
Amortized Cost	27,836	39,316
Gross Unrealized Gains	56	23
Gross Unrealized Losses	(3)	(33)
Estimated Fair Market Value	27,889	39,306
Foreign government securities
Short-term investments:
Amortized Cost		14,929
Gross Unrealized Gains		0
Gross Unrealized Losses		(14)
Estimated Fair Market Value		14,915
Certificates of deposit
Short-term investments:
Amortized Cost	1,245	4,711
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	(1)
Estimated Fair Market Value	1,245	4,711
Cash
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	194,955	105,484
Money market funds
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	872,431	541,575
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	35,009
Commercial paper
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	31,113	$ 3,999
Treasury bills
Cash and Cash Equivalents, at Carrying Value [Abstract]
Cash and cash equivalents	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