PENSION AND POSTRETIREMENT LIABILITIES: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2013
Components of net pension benefit costs:
Service cost	$ 3.3	$ 2.4
Interest cost	6.9	7.9
Expected return on assets	(3.6)	(3.9)
Amortization of actuarial losses	4.2	1.8
Amortization of prior service cost	0.2	0.2
Total pension cost	11.0	8.4
Expected employer contribution to defined benefit pension plans, including benefit payments paid directly to plan participants			25.8
Employer contributions	6.8
Defined benefit plan, expense recognised during the period	3.3	3.8
Multi-employer plan, expense recognised during the period	$ 1.8	$ 1.3