Debt Schedule of Convertible Debt (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended							12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% [Member]	Dec. 31, 2010 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% [Member]	Dec. 31, 2009 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% [Member]	Oct. 01, 2007 Senior Exchangeable Notes due 2012 at a fixed interest rate of 3.75% [Member]	Dec. 31, 2011 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50% [Member]	Dec. 31, 2010 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50% [Member]	Dec. 31, 2009 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50% [Member]	Aug. 01, 2009 Senior Exchangeable Notes due 2015 at a fixed interest rate of 4.50% [Member]
Schedule of Convertible Debt [Line Items]
Debt Instrument, Issuance Date				October 2007				August 2009
Debt Instrument, Convertible, Effective Interest Rate				6.21%	6.21%	6.21%		6.21%	6.21%	6.21%
Interest Expense, Debt, Excluding Amortization				$ 11,250	$ 11,250	$ 11,250		$ 7,763	$ 7,763	$ 2,911
Amortization of Debt Discount (Premium)	8,654	8,160	6,517	6,487	6,115	5,767		2,167	2,045	750
Debt Instrument, Convertible, Interest Expense	27,667	27,173	20,678
Debt Instrument, Interest Rate, Stated Percentage				3.75%			3.75%	4.50%			4.50%
Debt Instrument, Convertible, Terms of Conversion Feature				27.47 shares per $1,000 principal				29.97 shares per $1,000 principal
Debt Instrument, Convertible, Conversion Price				$ 36.41				$ 33.37
Option Indexed to Issuer's Equity, Indexed Shares				8,239,920				5,169,653
Conversion Premium				22.00%				22.50%
Effective Conversion Premium				40.00%				46.00%
Effective Conversion Price				$ 41.78				$ 39.85
Option Indexed to Issuer's Equity, Strike Price				$ 36.41				$ 33.37
Class of Warrant or Right, Exercise Price of Warrants or Rights				$ 41.78				$ 39.85
Debt Instrument Date of Maturity				2012				2015
Debt Instrument, Face Amount				300,000	300,000		300,000	172,500	172,500		172,500
Debt Instrument, Unamortized Discount				(5,378)	(11,865)			(8,975)	(11,142)
Convertible Debt	458,147	449,493
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 28,092	$ 28,092
Debt Instrument, Maturity Date				Oct. 15, 2012				Aug. 15, 2015