SHORT-TERM INVESTMENTS: (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Investments
Trading securities	$ 357,400,000		$ 357,400,000		$ 127,800,000
Weighted average interest rate (as a percent)			2.18%		1.87%
Available-for-sale	6,100,000		6,100,000		6,500,000
Weighted average interest rate (as a percent)			0.42%		0.43%
Total short-term investments	363,464,000		363,464,000		134,298,000
Trading securities:
Interest earned	1,900,000	900,000	3,900,000	2,500,000
Unrealized gain (loss)	(3,300,000)	3,700,000	(2,300,000)	8,800,000
Available-for-sale:
Investment redeemed		200,000	400,000	1,400,000
Maximum
Available-for-sale:
Interest earned	$ 100,000	$ 100,000	$ 100,000	$ 100,000