Financial Instruments and Fair Value Disclosures (Details 1) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
4.875% Senior Notes [Member]
Fair values and related carrying values of our debt instruments
Interest rate of senior notes	4.875%	4.875%
4.875% Senior Notes [Member] | Carrying Value [Member]
Fair values and related carrying values of our debt instruments
Senior notes	249.8	249.8
4.875% Senior Notes [Member] | Fair Value [Member]
Fair values and related carrying values of our debt instruments
Debt Instrument, Fair Value	271.9	272.9
5.15% Senior Notes [Member]
Fair values and related carrying values of our debt instruments
Interest rate of senior notes	5.15%	5.15%
5.15% Senior Notes [Member] | Carrying Value [Member]
Fair values and related carrying values of our debt instruments
Senior notes	249.8	249.8
5.15% Senior Notes [Member] | Fair Value [Member]
Fair values and related carrying values of our debt instruments
Debt Instrument, Fair Value	269.0	272.7
5.70% Senior Notes [Member]
Fair values and related carrying values of our debt instruments
Interest rate of senior notes	5.70%	5.70%
5.70% Senior Notes [Member] | Carrying Value [Member]
Fair values and related carrying values of our debt instruments
Senior notes	496.8	496.8
5.70% Senior Notes [Member] | Fair Value [Member]
Fair values and related carrying values of our debt instruments
Debt Instrument, Fair Value	588.5	555.0
5.875% Senior Notes [Member]
Fair values and related carrying values of our debt instruments
Interest rate of senior notes	5.875%	5.875%
5.875% Senior Notes [Member] | Carrying Value [Member]
Fair values and related carrying values of our debt instruments
Senior notes	499.4	499.4
5.875% Senior Notes [Member] | Fair Value [Member]
Fair values and related carrying values of our debt instruments
Debt Instrument, Fair Value	587.8	575.4