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, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Money market funds	$ 736,330	$ 608,225
Investment securities	132	138
Investment securities pledged as collateral	1,245,916	1,116,084
Prepaid forward contracts	7,317	3,385
Liabilities under derivative contracts:
Prepaid forward contracts	102,217	146,947
Level I
Assets:
Money market funds	736,330	608,225
Investment securities	132	138
Investment securities pledged as collateral	1,245,916	1,116,084
Prepaid forward contracts	0	0
Liabilities under derivative contracts:
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	7,317	3,385
Liabilities under derivative contracts:
Prepaid forward contracts	102,217	146,947
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 under derivative contracts:
Prepaid forward contracts	$ 0	$ 0