LEASES: (Details)	9 Months Ended
	Sep. 30, 2019 USD ($)
Leases
The weighted average remaining lease term	9 years
The weighted average discount rate	3.65%
Total lease expense	$ 87,000,000.0
Short term lease costs	300,000
Maturities of lease liabilities
2019	28,900,000
2020	115,400,000
2021	115,300,000
2022	112,200,000
2023	111,100,000
After 2023	929,200,000
Total lease payments	1,412,100,000
Less: interest on lease liabilities	(350,300,000)
Lease liabilities	1,061,800,000
Cost of sales (exclusive of depreciation, amortization and depletion)
Leases
Total lease expense	86,800,000
Selling, general and administrative
Leases
Total lease expense	100,000
Exploration
Leases
Total lease expense	$ 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