SHORT-TERM INVESTMENTS: (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Investments
Trading securities	$ 267,300,000		$ 202,600,000
Weighted average interest rate (as a percent)	1.87%		3.78%
Available for sale	5,700,000		5,700,000
Weighted average interest rate (as a percent)	0.41%		0.42%
Total short-term investments	273,010,000		208,268,000
Trading:
Interest earned	1,400,000	600,000
Unrealized gain (loss) at the end of the period	2,200,000	3,500,000
Maximum
Available-for-sale
Interest earned	$ 100,000	$ 100,000