Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Recurring basis [Abstract]
ARO Trust investments	$ 30.9	$ 17.8
Level 1 [Member]
Additional disclosures [Abstract]
Notes receivables	0	0
Long-term debt	0	0
Level 2 [Member]
Additional disclosures [Abstract]
Notes receivables	6.9	8.2
Long-term debt	(1,530.9)	(1,704.5)
Level 3 [Member]
Additional disclosures [Abstract]
Notes receivables	0	0
Long-term debt	0	0
Carrying Amount [Member]
Additional disclosures [Abstract]
Notes receivables	6.9	8.2
Long-term debt	(1,428.4)	(1,428.3)
Fair Value [Member]
Additional disclosures [Abstract]
Notes receivables	6.9	8.2
Long-term debt	(1,530.9)	(1,704.5)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Recurring basis [Abstract]
ARO Trust investments	30.9	17.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Recurring basis [Abstract]
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Recurring basis [Abstract]
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member]
Recurring basis [Abstract]
ARO Trust investments	30.9	17.8
Fair Value, Measurements, Recurring [Member] | Fair Value [Member]
Recurring basis [Abstract]
ARO Trust investments	$ 30.9	$ 17.8