Fair Value Measures Fair Value Measures (Schedule of Estimated Fair Value of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
$250 million senior notes due 2016 [member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Face Amount	$ 250,000
Long-tebt debt, fair value	275,313	276,250
$600 million senior notes due 2018 [member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Face Amount	600,000
Long-tebt debt, fair value	634,128	643,500
$500 million senior notes due 2021 [member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Face Amount	500,000
Long-tebt debt, fair value	$ 524,750	$ 516,250