Fair Value Measurements (Narrative) (Details)	Mar. 31, 2013 7.5% Senior Notes due 2018 [Member]	Dec. 31, 2012 7.5% Senior Notes due 2018 [Member]	Mar. 31, 2013 4.10% Senior Notes Due 2022 [Member]	Mar. 31, 2012 4.10% Senior Notes Due 2022 [Member]
Debt instrument , interest rate percentage	7.50%		4.10%	4.10%
Yield, Publicly-traded debt market	2.50%	2.60%	3.40%