Investment in Affiliate and Other Assets - Components of Net Capitalized Permit Costs (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Other Noncurrent Assets [Line Items]
Net capitalized permit costs	$ 53.2	$ 59.5
LLRW License - (expires in 2024) | Permits Subject to Amortization
Other Noncurrent Assets [Line Items]
Net capitalized permit costs	49.6	54.0
Byproduct License - (expires in 2018) | Permits Subject to Amortization
Other Noncurrent Assets [Line Items]
Net capitalized permit costs	3.5	4.6
Other - (expires 2014 - 2024) | Permits Subject to Amortization
Other Noncurrent Assets [Line Items]
Net capitalized permit costs	$ 0.1	$ 0.9