Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Cash Flow Hedge Asset, at Fair Value	$ 2.1
Natural Gas [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Cash Flow Hedge Asset, at Fair Value	2.1
Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Cash Flow Hedge Asset, at Fair Value	0
Natural Gas [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Cash Flow Hedge Asset, at Fair Value	2.1
Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Price Risk Cash Flow Hedge Asset, at Fair Value	0
Senior notes due 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	256.3	258.1
Senior Notes, Noncurrent	250.0	250.0
Senior notes due 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	232.9	244.4
Senior Notes, Noncurrent	222.3	222.1
Senior notes due 2022 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	400.0	424.0
Senior Notes, Noncurrent	$ 405.1	$ 405.4