ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		$ 10,636,343		$ 11,260,598
Tax (benefit) associated with gain (loss) on components of foreign currency translation		(16,023)	$ (19,733)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[1]	45,176		(15,700)
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		56,406		236,982
Net Investment Hedging [Member] | Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent		(80,468)		(64,445)
Net Investment Hedging [Member] | Foreign currency translation adjustments | Euro Senior Notes
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		273,400		190,400
Stockholders' equity, before tax		346,700		237,200
Net Investment Hedging [Member] | Foreign currency translation adjustments | Foreign Currency Forward
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(35,000)
Stockholders' equity, before tax		(52,600)
Ctrip equity securities | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[2]	0		241,088
Stockholders' equity, before tax				298,700
AOCI Tax, Attributable to Parent		0		57,597
Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity	[3]	11,230		11,594
Stockholders' equity, before tax		42,700		44,500
AOCI Tax, Attributable to Parent		31,475		32,903
Currency translation adjustment on deemed repatriation tax liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax (benefit) associated with gain (loss) on components of foreign currency translation		$ 10,500	$ 0
Internal Revenue Service (IRS)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Statutory federal rate		21.00%	35.00%
Internal Revenue Service (IRS) | Ctrip equity securities | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent				62,900
Internal Revenue Service (IRS) | Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI Tax, Attributable to Parent		$ 7,600		$ 500
Tax and Customs Administration, Netherlands
Accumulated Other Comprehensive Income (Loss) [Line Items]
Statutory federal rate		25.00%		25.00%
Tax and Customs Administration, Netherlands | Ctrip equity securities | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity, before tax, tax-exempt				$ 319,900
Stockholders' equity, before tax, taxable				(21,200)
AOCI Tax, Attributable to Parent				(5,300)
Tax and Customs Administration, Netherlands | Debt Securities [Member] | Net unrealized gains/(losses) on marketable securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity, before tax, tax-exempt		$ (52,900)		(85,300)
Stockholders' equity, before tax, taxable		95,600		129,800
AOCI Tax, Attributable to Parent		$ 23,900		$ 32,400

[1] (1) Foreign currency translation adjustments, net of tax, at March 31, 2018 and December 31, 2017, include accumulated net losses from fair value adjustments of $35.0 million after tax ($52.6 million before tax) associated with previously settled derivatives that were designated as net investment hedges. Foreign currency translation adjustments, net of tax, include foreign currency transaction losses of $273.4 million after tax ($346.7 million before tax) and $190.4 million after tax ($237.2 million before tax) at March 31, 2018 and December 31, 2017, respectively, associated with the Company's Euro-denominated debt. The Company's Euro-denominated debt is designated as a hedge against the impact of currency fluctuations on its Euro-denominated net assets (see Note 8). The remaining balance in foreign currency translation adjustments relates to the cumulative impacts of currency fluctuations on the Company's international non-U.S. Dollar denominated net assets. During the three months ended March 31, 2018, the Company recorded deferred tax charges of $10.5 million related to its one-time deemed repatriation tax liability recorded at December 31, 2017 and current year foreign earnings subject to U.S. federal and state income tax, resulting from the introduction of the Tax Act. Prior to January 1, 2018, foreign currency translation adjustments excluded U.S. federal and state income taxes as a result of the Company's intention to indefinitely reinvest the earnings of its international subsidiaries outside of the United States.

[2] (2) Net unrealized gains on marketable equity securities, net of tax, at December 31, 2017 related to changes in the fair value of the Company's investment in Ctrip equity securities (see Note 5). Net unrealized gains before tax on equity securities at December 31, 2017 were $298.7 million, of which unrealized gains of $319.9 million were exempt from tax in the Netherlands. Unrealized losses of $21.2 million were taxable at a 25% tax rate in the Netherlands, which resulted in a tax benefit of $5.3 million at December 31, 2017. The Company also recorded U.S. tax charges of $62.9 million at December 31, 2017 related to these investments. Changes in fair value subsequent to January 1, 2018 are recognized in net income (see Note 1).

[3] (3) Net unrealized gains on marketable debt securities, net of tax, relates to changes in the fair value of the Company's investments in debt securities (see Note 5). Net unrealized gains before tax on debt securities at March 31, 2018 and December 31, 2017 were $42.7 million and $44.5 million, respectively, of which unrealized losses of $52.9 million and $85.3 million, respectively, were exempt from tax in the Netherlands. Unrealized gains of $95.6 million and $129.8 million at March 31, 2018 and December 31, 2017, respectively, were taxable at a 25% tax rate in the Netherlands, resulting in tax charges of $23.9 million and $32.4 million, respectively. The Company also recorded U.S. tax charges of $7.6 million and $0.5 million at March 31, 2018 and December 31, 2017, respectively, related to these investments.