LEASES: (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Leases
Lessee, operating lease, option to extend	false
Lessee, operating lease, option to terminate	false
The weighted average remaining lease term	9 years
The weighted average discount rate	3.64%
Total lease expense	$ 57,700,000	$ 57,800,000
Maturities of lease liabilities
2020	57,700,000
2021	115,400,000
2022	112,600,000
2023	111,400,000
2024	103,600,000
After 2024	826,300,000
Total lease payments	1,327,000,000.0
Less: interest on lease liabilities	(315,800,000)
Lease liabilities	1,011,200,000
Cost of sales (exclusive of depreciation, amortization and depletion)
Leases
Total lease expense	57,500,000	57,600,000
Selling, general and administrative
Leases
Total lease expense	$ 100,000	100,000
Minimum
Leases
Remaining lease terms	2 years
Maximum
Leases
Underlying asset value	$ 10,000
Total nominal contract value	$ 100,000
Remaining lease terms	13 years
Short term lease costs	$ 100,000
Maximum | Exploration
Leases
Total lease expense	$ 100,000	$ 100,000