Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2012

Components of Net Capitalized Permit Costs

The components of net capitalized permit costs are presented in the table below.

	December 31,
	2011		2012
	(In millions)
Net permit costs for:
Pending renewals of prior permits	$	.3	$	.2
Issued permits which are being amortized:
LLRW license (expires in 2024)		—		58.3
Byproduct license (expires in 2018)		6.6		5.6
Other (expires 2013 – 2024)		1.6		1.6
Total pending renewals and issued permits which are being amortized		8.5		65.7
Issued permits not yet subject to amortization		58.1		—
Total	$	66.6	$	65.7

Computation of Depreciation of Property and Equipment

We compute depreciation of property and equipment for financial reporting purposes (including mining equipment) principally by the straight-line method over the estimated useful lives of the assets as follows:

Asset	Useful lives
Buildings and improvements	10 to 40 years
Machinery and equipment	3 to 20 years
Mine development costs	Units-of-production
Landfill disposal costs	15 to 39 years