SHORT-TERM INVESTMENTS: (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Investments
Trading securities	$ 202,600,000	$ 127,800,000
Weighted average interest rate (as a percent)	3.78%	1.87%
Available-for-sale	5,700,000	6,500,000
Weighted average interest rate (as a percent)	0.42%	0.43%
Total short-term investments	208,268,000	134,298,000
Trading securities:
Interest earned	5,200,000	3,100,000
Unrealized gain (loss)	(1,900,000)	2,400,000
Available-for-sale
Interest earned		100,000
Investment redeemed	800,000	1,900,000
Contractual maturities of the Company's available for sale debt securities
One year or less	400,000	400,000
Maturing after five years through ten years	200,000
Due after 10 years	5,100,000	6,100,000
Total debt securities	5,700,000	6,500,000
Maximum
Available-for-sale
Interest earned	$ 100,000