DEBT (Outstanding Debt) (Details) $ / shares in Units, € in Millions, $ in Millions		1 Months Ended	12 Months Ended
	Aug. 20, 2014 USD ($) Days $ / shares	May 31, 2013 USD ($) Days $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Aug. 15, 2017 USD ($)	Jul. 24, 2017 USD ($)	Mar. 10, 2017 EUR (€)	May 23, 2016 USD ($)	Nov. 25, 2015 EUR (€)	Mar. 13, 2015 USD ($)	Mar. 03, 2015 EUR (€)	Sep. 23, 2014 EUR (€)	Jun. 30, 2013 USD ($)	Mar. 31, 2012 USD ($) $ / shares
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 8,787.0		$ 9,003.0
Unamortized Debt Discount and Debt Issuance Cost										(138.0)		(193.0)
Long-term Debt										8,649.0		8,810.0
Description of Senior Notes
Reclassification of unamortized debt discount from additional paid-in-capital to convertible debt in mezzanine										0.0		3.0
Payments for conversion of senior notes			$ 1,487.0	$ 286.0	$ 0.0
Amortization of debt discount			52.0	70.0	69.0
Level 2
Description of Senior Notes
Estimated market value of outstanding senior notes										9,300.0		11,100.0
Momondo Group
Description of Senior Notes
Debt assumed in acquisition(s)														$ 15.0
1.00% Convertible Senior Notes Due March 2018
Debt Instrument [Line Items]
Outstanding Principal Amount												714.0
Unamortized Debt Discount and Debt Issuance Cost												(3.0)
Short-term Debt												$ 711.0
Description of Senior Notes
Reclassification of unamortized debt discount from additional paid-in-capital to convertible debt in mezzanine										3.0
Senior notes face amount																						$ 1,000.0
Interest rate stated percentage (as a percent)											1.00%	1.00%										1.00%
Debt Instrument, Convertible, Conversion Price | $ / shares																						$ 944.61
Payments for conversion of senior notes			714.0
Cash payment of the conversion value in excess of the principal amount			773.0
Debt Instrument, Interest Rate, Effective Percentage																						3.50%
Debt discount related to convertible notes, net of tax																						$ 81.0
Debt discount related to convertible notes, before tax																						$ 135.0
Convertible Notes
Description of Senior Notes
Interest Expense, Debt			66.0	94.0	95.0
Debt Instrument, Coupon Interest Expense			14.0	21.0	22.0
Amortization of debt discount			$ 50.0	$ 68.0	$ 68.0
Weighted-average effective interest rate during the period			3.20%	3.40%	3.40%
Amortization of debt issuance costs included in interest expense			$ 2.0	$ 5.0	$ 5.0
0.35% Senior Convertible Notes Due June 2020
Debt Instrument [Line Items]
Outstanding Principal Amount										1,000.0		$ 1,000.0
Unamortized Debt Discount and Debt Issuance Cost										(39.0)		(65.0)
Long-term Debt										$ 961.0		$ 935.0
Description of Senior Notes
Senior notes face amount		$ 1,000.0
Interest rate stated percentage (as a percent)		0.35%							0.35%	0.35%	0.35%	0.35%
Unamortized Debt Discount		$ 20.0
Payments of debt issuance costs								$ 1.0
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 1,315.10
Ratio of closing share price to conversion price as a condition for conversion of convertible 2015 Senior Notes, minimum (as a percent)		150.00%
Debt Instrument, Interest Rate, Effective Percentage		3.13%
Debt discount related to convertible notes, net of tax																					$ 92.0
Debt discount related to convertible notes, before tax																					$ 154.0
Amortization of debt discount			3.0	3.0	3.0
0.35% Senior Convertible Notes Due June 2020 | Minimum
Description of Senior Notes
Debt Instrument, Convertible, Threshold Consecutive Trading Days | Days		20
Additional Payment To Debt Holder Settled In Shares Aggregate Value Of Shares		$ 0.0
0.35% Senior Convertible Notes Due June 2020 | Maximum
Description of Senior Notes
Debt Instrument, Convertible, Threshold Consecutive Trading Days | Days		30
Additional Payment To Debt Holder Settled In Shares Aggregate Value Of Shares		$ 397.0
0.9% Senior Convertible Notes Due September 2021
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 1,000.0		$ 1,000.0
Unamortized Debt Discount and Debt Issuance Cost										(61.0)		(83.0)
Long-term Debt										$ 939.0		$ 917.0
Description of Senior Notes
Senior notes face amount	$ 1,000.0
Interest rate stated percentage (as a percent)	0.90%								0.90%	0.90%	0.90%	0.90%
Payments of debt issuance costs							$ 11.0
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 2,055.50
Ratio of closing share price to conversion price as a condition for conversion of convertible 2015 Senior Notes, minimum (as a percent)	150.00%
Debt Instrument, Interest Rate, Effective Percentage	3.18%
Debt discount related to convertible notes, net of tax						$ 83.0
Debt discount related to convertible notes, before tax						143.0
0.9% Senior Convertible Notes Due September 2021 | Minimum
Description of Senior Notes
Debt Instrument, Convertible, Threshold Consecutive Trading Days | Days	20
Additional Payment To Debt Holder Settled In Shares Aggregate Value Of Shares	$ 0.0
0.9% Senior Convertible Notes Due September 2021 | Maximum
Description of Senior Notes
Debt Instrument, Convertible, Threshold Consecutive Trading Days | Days	30
Additional Payment To Debt Holder Settled In Shares Aggregate Value Of Shares	$ 375.0
0.8% Senior Notes Due March 2022
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 1,143.0		$ 1,201.0
Unamortized Debt Discount and Debt Issuance Cost										(5.0)		(6.0)
Long-term Debt										$ 1,138.0		$ 1,195.0
Description of Senior Notes
Senior notes face amount | €									€ 1,000.0		€ 1,000.0				€ 1,000.0
Interest rate stated percentage (as a percent)									0.80%	0.80%	0.80%	0.80%			0.80%
Unamortized Debt Discount | €															€ 2.0
Payments of debt issuance costs				5.0
Debt Instrument, Interest Rate, Effective Percentage															0.84%
2.15% Senior Notes Due November 2022
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 858.0		$ 900.0
Unamortized Debt Discount and Debt Issuance Cost										(4.0)		(5.0)
Long-term Debt										$ 854.0		$ 895.0
Description of Senior Notes
Senior notes face amount | €									€ 750.0		€ 750.0						€ 750.0
Interest rate stated percentage (as a percent)									2.15%	2.15%	2.15%	2.15%					2.15%
Unamortized Debt Discount | €																	€ 2.0
Payments of debt issuance costs						4.0
Debt Instrument, Interest Rate, Effective Percentage																		2.20%
2.75% Senior Notes Due March 2023
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 500.0		$ 500.0
Unamortized Debt Discount and Debt Issuance Cost										(3.0)		(3.0)
Long-term Debt										$ 497.0		$ 497.0
Description of Senior Notes
Senior notes face amount													$ 500.0
Interest rate stated percentage (as a percent)									2.75%	2.75%	2.75%	2.75%	2.75%
Unamortized Debt Discount													$ 1.0
Payments of debt issuance costs				3.0
Debt Instrument, Interest Rate, Effective Percentage													2.78%
2.375% Senior Notes Due September 2024
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 1,143.0		$ 1,201.0
Unamortized Debt Discount and Debt Issuance Cost										(10.0)		(12.0)
Long-term Debt										$ 1,133.0		$ 1,189.0
Description of Senior Notes
Senior notes face amount | €									€ 1,000.0		€ 1,000.0									€ 1,000.0
Interest rate stated percentage (as a percent)									2.375%	2.375%	2.375%	2.375%								2.375%
Unamortized Debt Discount | €																				€ 9.0
Payments of debt issuance costs							$ 7.0
Debt Instrument, Interest Rate, Effective Percentage																				2.48%
3.65% Senior Notes Due March 2025
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 500.0		$ 500.0
Unamortized Debt Discount and Debt Issuance Cost										(3.0)		(3.0)
Long-term Debt										$ 497.0		$ 497.0
Description of Senior Notes
Senior notes face amount																		$ 500.0
Interest rate stated percentage (as a percent)									3.65%	3.65%	3.65%	3.65%						3.65%
Unamortized Debt Discount																		$ 1.0
Payments of debt issuance costs						3.0
Debt Instrument, Interest Rate, Effective Percentage																		3.68%
3.6% Senior Notes Due June 2026
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 1,000.0		$ 1,000.0
Unamortized Debt Discount and Debt Issuance Cost										(6.0)		(7.0)
Long-term Debt										$ 994.0		$ 993.0
Description of Senior Notes
Senior notes face amount																$ 1,000.0
Interest rate stated percentage (as a percent)									3.60%	3.60%	3.60%	3.60%				3.60%
Unamortized Debt Discount																$ 2.0
Payments of debt issuance costs					6.0
Debt Instrument, Interest Rate, Effective Percentage																3.62%
1.8% Senior Notes Due March 2027
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 1,143.0		$ 1,201.0
Unamortized Debt Discount and Debt Issuance Cost										(4.0)		(5.0)
Long-term Debt										$ 1,139.0		$ 1,196.0
Description of Senior Notes
Senior notes face amount | €									€ 1,000.0		€ 1,000.0								€ 1,000.0
Interest rate stated percentage (as a percent)									1.80%	1.80%	1.80%	1.80%							1.80%
Unamortized Debt Discount | €																			€ 0.3
Payments of debt issuance costs						$ 6.0
Debt Instrument, Interest Rate, Effective Percentage																			1.80%
3.55% Senior Notes Due March 2028
Debt Instrument [Line Items]
Outstanding Principal Amount										$ 500.0		$ 500.0
Unamortized Debt Discount and Debt Issuance Cost										(3.0)		(4.0)
Long-term Debt										$ 497.0		$ 496.0
Description of Senior Notes
Senior notes face amount													$ 500.0
Interest rate stated percentage (as a percent)									3.55%	3.55%	3.55%	3.55%	3.55%
Unamortized Debt Discount													$ 0.4
Payments of debt issuance costs				3.0
Debt Instrument, Interest Rate, Effective Percentage													3.56%
Other Long-term Debt
Description of Senior Notes
Interest Expense, Debt			170.0	145.0	108.0
Debt Instrument, Coupon Interest Expense			$ 163.0	$ 139.0	$ 104.0