FAIR VALUE MEASUREMENT, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value Measured on a Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets:
Money market funds	$ 922,765	$ 736,330
Investment securities	130	132
Investment securities pledged as collateral	1,211,982	1,245,916
Prepaid forward contracts	82,408	7,317
Liabilities:
Prepaid forward contracts	2,706	102,217
Level I
Assets:
Money market funds	922,765	736,330
Investment securities	130	132
Investment securities pledged as collateral	1,211,982	1,245,916
Prepaid forward contracts	0	0
Liabilities:
Prepaid forward contracts	0	0
Level II
Assets:
Money market funds	0	0
Investment securities	0	0
Investment securities pledged as collateral	0	0
Prepaid forward contracts	82,408	7,317
Liabilities:
Prepaid forward contracts	2,706	102,217
Level III
Assets:
Money market funds	0	0
Investment securities	0	0
Investment securities pledged as collateral	0	0
Prepaid forward contracts	0	0
Liabilities:
Prepaid forward contracts	$ 0	$ 0