Fair Value Measurements (Schedule of Fair Value of Debt Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Feb. 28, 2012	Nov. 30, 2011	Jun. 30, 2011	Nov. 30, 2010
Eight Point Six Two Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Senior Notes	$ 600,000	$ 600,000	$ 200,000	$ 200,000	$ 400,000	$ 400,000
Seven Point Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Senior Notes	300,000	300,000
Four Point Three Seven Five Percent Convertible Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Convertible debt	4,425	73,750
Portion at Fair Value, Fair Value Disclosure [Member] | Eight Point Six Two Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Fair value of Senior Notes	640,140	645,000
Portion at Fair Value, Fair Value Disclosure [Member] | Seven Point Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Fair value of Senior Notes	312,000	308,250
Portion at Fair Value, Fair Value Disclosure [Member] | Four Point Three Seven Five Percent Convertible Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Fair Value of Convertible Senior Notes	4,436	73,842
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Eight Point Six Two Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Senior Notes	595,479	595,151
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Seven Point Five Percent Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Senior Notes	300,000	300,000
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Four Point Three Seven Five Percent Convertible Senior Notes [Member]
Schedule of Fair Value of Debt Instruments [Line Items]
Convertible debt	$ 4,425	$ 72,657