INVESTMENTS - HTM & AFS (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Held to Maturity and Available-for-sale
Held to maturity and available-for-sale securities, Amortized Cost	$ 759,958	$ 820,164
Held to maturity and available-for-sale securities, Gross Unrealized Holding Gains	63	115
Held to maturity and available-for-sale securities, Gross Unrealized Holding Losses	296	3
Held to maturity and available-for-sale securities, Fair Value	759,725	820,276
Held to maturity and available-for-sale securities, Continuous Unrealized Loss Position less than 12 Months	296
Held to Maturity, Available-for-sale, and Trading
Investments, Fair Value Disclosure	759,725	824,186
Short-term | Commercial paper
Held-to-Maturity
Amortized Cost	3,978	19,482
Gross Unrealized Holding Losses		2
Fair Value	3,978	19,480
Short-term | Municipal securities
Held-to-Maturity
Amortized Cost	709,207	744,542
Gross Unrealized Holding Gains	63	105
Gross Unrealized Holding Losses	192
Fair Value	709,078	744,647
Continuous Unrealized Loss Position less than 12 Months	192
Short-term | U.S. government agency securities
Held-to-Maturity
Amortized Cost	23,369	9,199
Gross Unrealized Holding Losses	58	1
Fair Value	23,311	9,198
Continuous Unrealized Loss Position less than 12 Months	58
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost		4,001
Fair Value		4,001
Short-term | Auction rate securities
Trading
Fair Value		3,910
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	42,940
Gross Unrealized Holding Gains		10
Gross Unrealized Holding Losses	8
Fair Value	11,063	$ 42,950
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