Cash and Available-for-Sale Securities - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,064,018	$ 601,035
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1,064,018	601,035
Cash and cash equivalents	499,018	386,035	$ 252,615	$ 365,963
Investments	565,000	215,000
Cash
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	281,047	225,235
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	281,047	225,235
Cash and cash equivalents	281,047	225,235
Investments	0	0
Time deposits
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	590,000	235,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	590,000	235,000
Cash and cash equivalents	25,000	20,000
Investments	565,000	215,000
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	192,971	140,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	192,971	140,800
Cash and cash equivalents	192,971	140,800
Investments	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Sum of the carrying amounts as of the balance sheet date of all investments.

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