Summary of significant accounting policies - Components of Net Capitalized Permit Costs (Detail) (Capitalized Operating Permit Costs [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	$ 65.7	$ 66.6
Pending Renewals Of Prior Permits [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	0.2	0.3
Permits Subject to Amortization [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	65.7	8.5
Permits Not Yet Subject to Amortization [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs		58.1
Byproduct License - (expires in 2018) [Member] | Permits Subject to Amortization [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	5.6	6.6
Other - (expires 2013 - 2024) [Member] | Permits Subject to Amortization [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	1.6	1.6
LLRW License - (expires in 2024) [Member] | Permits Subject to Amortization [Member]
Schedule Of Significant Accounting Policies [Line Items]
Net capitalized permit costs	$ 58.3