INVESTMENTS - HTM & AFS (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Held to Maturity and Available-for-sale
Held to maturity and available-for-sale securities, Amortized Cost	$ 515,662	$ 759,958
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains	24	63
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses	163	296
Held to maturity and available-for-sale securities, Fair Value	515,523	759,725
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months	163	296
Held to Maturity, Available-for-sale, and Trading
Investments, Fair Value Disclosure	515,523	759,725
Short-term | Commercial paper
Held-to-Maturity
Amortized Cost	7,969	3,978
Fair Value	7,969	3,978
Short-term | Municipal securities
Held-to-Maturity
Amortized Cost	464,865	709,207
Gross Unrealized Holding Gains	22	63
Gross Unrealized Holding Losses	148	192
Fair Value	464,739	709,078
Continuous Unrealized Loss Position less than 12 Months	148	192
Short-term | U.S. government agency securities
Held-to-Maturity
Amortized Cost	17,316	23,369
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	7	58
Fair Value	17,310	23,311
Continuous Unrealized Loss Position less than 12 Months	7	58
Short-term | U.S. Treasuries
Held-to-Maturity
Amortized Cost	18,093	8,056
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses		13
Fair Value	18,094	8,043
Continuous Unrealized Loss Position less than 12 Months		13
Long-term | Municipal securities
Held-to-Maturity
Amortized Cost	7,419	11,071
Gross Unrealized Holding Losses	8	8
Fair Value	7,411	11,063
Continuous Unrealized Loss Position less than 12 Months	$ 8	8
Long-term | U.S. government agency securities
Held-to-Maturity
Amortized Cost		4,277
Gross Unrealized Holding Losses		25
Fair Value		4,252
Continuous Unrealized Loss Position less than 12 Months		$ 25