SHORT-TERM INVESTMENTS: (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Investments
Trading securities	$ 386,800,000		$ 386,800,000		$ 127,800,000
Weighted average interest rate (as a percent)			2.05%		1.87%
Available for sale	6,100,000		6,100,000		6,500,000
Weighted average interest rate (as a percent)			0.41%		0.43%
Total short-term investments	392,933,000		392,933,000		134,298,000
Trading securities:
Interest earned	1,400,000	900,000	2,000,000	1,600,000
Unrealized gain (loss)	(100,000)	100,000	1,000,000	5,100,000
Available-for-sale:
Investment redeemed	400,000	900,000	400,000	1,200,000
Maximum
Available-for-sale:
Interest earned	$ 100,000	$ 100,000	$ 100,000	$ 100,000