INVESTMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Held to Maturity and Available-for-sale
Held to maturity and available-for-sale securities, Amortized Cost	$ 1,283,976	$ 820,164
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains	148		$ 115
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses	57	3
Held to maturity and available-for-sale securities, Fair Value	1,284,067	820,276
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months	57
Held to Maturity, Available-for-sale, and Trading
Investments, Fair Value Disclosure	1,287,313	824,186
U.S. Treasuries
Held-to-Maturity
Amortized Cost	74,981
Gross Unrealized Holding Gains	6
Fair Value	74,987
Held to Maturity, Available-for-sale, and Trading
Investments, Fair Value Disclosure	74,987
Certificates of deposit
Held-to-Maturity
Amortized Cost	32,001
Fair Value	32,001
Held to Maturity, Available-for-sale, and Trading
Investments, Fair Value Disclosure	32,001
Variable rate demand notes
Held-to-Maturity
Amortized Cost		4,001
Fair Value		4,001
Available-for-sale
Amortized Cost	3,901
Fair Value	3,901
Short-term | Commercial paper
Held-to-Maturity
Amortized Cost	182,130	19,482
Gross Unrealized Holding Losses		2
Fair Value	182,130	19,480
Short-term | Municipal securities
Held-to-Maturity
Amortized Cost	587,740	744,542
Gross Unrealized Holding Gains	82	105
Gross Unrealized Holding Losses	42
Fair Value	587,780	744,647
Continuous Unrealized Loss Position less than 12 Months	42
Short-term | U.S. government agency securities
Held-to-Maturity
Amortized Cost	350,859	9,199
Gross Unrealized Holding Gains	27
Gross Unrealized Holding Losses	10	1
Fair Value	350,876	9,198
Continuous Unrealized Loss Position less than 12 Months	10
Short-term | Auction rate securities
Trading
Fair Value	3,246	3,910
Long-term | Municipal securities
Held-to-Maturity
Amortized Cost	52,364	42,940
Gross Unrealized Holding Gains	33	10
Gross Unrealized Holding Losses	5
Fair Value	52,392	$ 42,950
Continuous Unrealized Loss Position less than 12 Months	$ 5