SHORT-TERM INVESTMENTS: (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Investments
Trading securities	$ 39.1		$ 39.1		$ 49.2
Weighted average interest rate (as a percent)			1.60%		2.20%
Available-for-sale	1.1		$ 1.1		$ 2.1
Weighted average interest rate (as a percent)			0.70%		0.78%
Total	40.2		$ 40.2		$ 51.3
Trading securities:
Interest earned	0.2	$ 0.3	0.4	$ 0.7
Unrealized gain (loss) at the end of the period		0.4	(0.1)	0.4
Available-for-sale:
Investment redeemed	1.0	0.2	1.0	0.5
Maximum
Trading securities:
Unrealized gain (loss) at the end of the period	0.1
Available-for-sale:
Interest earned	$ 0.1	$ 0.1	$ 0.1	$ 0.1