INVESTMENTS - HTM & AFS (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Held to Maturity and Available-for-sale
Held to maturity and available-for-sale securities, Amortized Cost	$ 44,319	$ 759,958
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains	5	63
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses	7	296
Held to maturity and available-for-sale securities, Fair Value	44,317	759,725
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months	7	296
Short-term | Commercial paper
Held-to-Maturity
Amortized Cost		3,978
Fair Value		3,978
Short-term | Municipal securities
Held-to-Maturity
Amortized Cost	44,319	709,207
Gross Unrealized Holding Gains	5	63
Gross Unrealized Holding Losses	7	192
Fair Value	44,317	709,078
Continuous Unrealized Loss Position less than 12 Months	$ 7	192
Short-term | U.S. government agency securities
Held-to-Maturity
Amortized Cost		23,369
Gross Unrealized Holding Losses		58
Fair Value		23,311
Continuous Unrealized Loss Position less than 12 Months		58
Short-term | U.S. Treasuries
Held-to-Maturity
Amortized Cost		8,056
Gross Unrealized Holding Losses		13
Fair Value		8,043
Continuous Unrealized Loss Position less than 12 Months		13
Long-term | Municipal securities
Held-to-Maturity
Amortized Cost		11,071
Gross Unrealized Holding Losses		8
Fair Value		11,063
Continuous Unrealized Loss Position less than 12 Months		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