Cumulative Effect Of New Accounting Principle In Period Of Adoption, Before Tax

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Period of time between issuance and maturity of debt investment, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Debt Securities, Available-For-Sale [Abstract]

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Equity Securities, FV-NI [Abstract]

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Change in unrealized Gains/(Losses) on equity securities in the period.

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of investment in equity security without readily determinable fair value.

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Cost of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale.

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Amount of gross unrealized gains on investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale.

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Amount of gross unrealized losses on investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale.

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Maximum Ownership Percentage in Ctrip

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Weighted maturity of investments

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

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Total debt and equity financial instruments including: (1) securities held-to-maturity and (2) securities available-for-sale that will be held for the long-term.

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INVESTMENTS (Details) - USD ($) $ in Thousands					3 Months Ended
INVESTMENTS (Details) - USD ($) $ in Thousands	Sep. 12, 2016	Dec. 11, 2015	May 26, 2015	Aug. 07, 2014	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Marketable securities:
Short-term investments in marketable securities					$ 4,240,704			$ 4,859,873
Long-term investments in marketable securities					9,432,763			10,421,600
Investments [Abstract]
Cumulative effect of adoption of accounting standard updates					188,539
Net unrealized gains on marketable equity securities					$ 54,514	$ 0
Weighted average life of all fixed income investments, excluding the Company's investment in Ctrip convertible debt securities					1 year 7 months
Equity Investments without Readily Determinable Fair Value					$ 450,900
Cost Method Investments								450,900
Ctrip.com International, Ltd.
Investments [Abstract]
Debt Investment, Term	6 years	10 years	5 years	5 years
Maximum Ownership Percentage in Ctrip			15.00%
Short-term Investments
Marketable securities:
Cost					4,259,603
Gross Unrealized Gains					312
Gross Unrealized Losses					(19,211)
Short-term investments in marketable securities					4,240,704
Available-for-sale securities:
Cost								4,866,475
Gross Unrealized Gains								700
Gross Unrealized Losses								(7,302)
Fair Value								4,859,873
Short-term Investments \| International government securities
Debt securities:
Cost					461,389
Gross Unrealized Gains					56
Gross Unrealized Losses					(555)
Fair Value					460,890
Available-for-sale securities:
Cost								725,566
Gross Unrealized Gains								246
Gross Unrealized Losses								(436)
Fair Value								725,376
Short-term Investments \| U.S. government securities
Debt securities:
Cost					794,489
Gross Unrealized Gains					0
Gross Unrealized Losses					(4,064)
Fair Value					790,425
Available-for-sale securities:
Cost								996,112
Gross Unrealized Gains								5
Gross Unrealized Losses								(1,999)
Fair Value								994,118
Short-term Investments \| Corporate debt securities
Debt securities:
Cost					2,952,709
Gross Unrealized Gains					256
Gross Unrealized Losses					(14,565)
Fair Value					2,938,400
Available-for-sale securities:
Cost								3,067,703
Gross Unrealized Gains								449
Gross Unrealized Losses								(4,837)
Fair Value								3,063,315
Short-term Investments \| U.S government agency securities
Debt securities:
Cost					4,435
Gross Unrealized Gains					0
Gross Unrealized Losses					(27)
Fair Value					4,408
Available-for-sale securities:
Cost								4,444
Gross Unrealized Gains								0
Gross Unrealized Losses								(30)
Fair Value								4,414
Short-term Investments \| Commercial paper
Debt securities:
Cost					46,581
Gross Unrealized Gains					0
Gross Unrealized Losses					0
Fair Value					46,581
Available-for-sale securities:
Cost								72,650
Gross Unrealized Gains								0
Gross Unrealized Losses								0
Fair Value								72,650
Long-term Investments
Marketable securities:
Cost					9,018,133
Gross Unrealized Gains					498,244
Gross Unrealized Losses					(83,614)
Long-term investments in marketable securities					9,432,763
Available-for-sale securities:
Cost								10,072,468
Gross Unrealized Gains								412,786
Gross Unrealized Losses								(63,654)
Fair Value								10,421,600
Long-term Investments \| International government securities
Debt securities:
Cost					786,958
Gross Unrealized Gains					1,760
Gross Unrealized Losses					(584)
Fair Value					788,134
Available-for-sale securities:
Cost								607,000
Gross Unrealized Gains								1,588
Gross Unrealized Losses								(678)
Fair Value								607,910
Long-term Investments \| U.S. government securities
Debt securities:
Cost					555,413
Gross Unrealized Gains					0
Gross Unrealized Losses					(11,525)
Fair Value					543,888
Available-for-sale securities:
Cost								844,910
Gross Unrealized Gains								2
Gross Unrealized Losses								(10,636)
Fair Value								834,276
Long-term Investments \| Corporate debt securities
Debt securities:
Cost					5,744,951
Gross Unrealized Gains					7,272
Gross Unrealized Losses					(71,499)
Fair Value					5,680,724
Available-for-sale securities:
Cost								6,689,747
Gross Unrealized Gains								8,399
Gross Unrealized Losses								(41,722)
Fair Value								6,656,424
Long-term Investments \| U.S government agency securities
Debt securities:
Cost					500
Gross Unrealized Gains					0
Gross Unrealized Losses					(6)
Fair Value					494
Available-for-sale securities:
Cost								500
Gross Unrealized Gains								0
Gross Unrealized Losses								(6)
Fair Value								494
Long-term Investments \| Ctrip convertible debt securities \| Ctrip.com International, Ltd.
Debt securities:
Cost	$ 25,000	$ 500,000	$ 250,000	$ 500,000	1,275,000
Gross Unrealized Gains					136,013
Gross Unrealized Losses					0
Fair Value					1,411,013
Available-for-sale securities:
Cost								1,275,000
Gross Unrealized Gains								103,100
Gross Unrealized Losses								(9,600)
Fair Value								1,368,500
Long-term Investments \| Ctrip equity securities \| Ctrip.com International, Ltd.
Equity securities:
Cost					655,311
Gross Unrealized Gains					353,199
Gross Unrealized Losses					0
Fair Value					1,008,510
Available-for-sale securities:
Cost								655,311
Gross Unrealized Gains								299,697
Gross Unrealized Losses								(1,012)
Fair Value								$ 953,996
Retained Earnings
Investments [Abstract]
Cumulative effect of adoption of accounting standard updates					$ 429,627
Accounting Standards Update 2016-01 \| Retained Earnings
Investments [Abstract]
Cumulative Effect on Retained Earnings, before Tax							$ 298,700
Cumulative effect of adoption of accounting standard updates							$ 241,100