Debt (Narrative) (Details) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Apr. 29, 2013 Subsequent Event [Member]	Mar. 31, 2013 Senior Secured Revolving Credit Facility [Member]	Dec. 31, 2012 Senior Secured Revolving Credit Facility [Member]	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2013 Convertible Senior Notes [Member]	Dec. 31, 2012 Convertible Senior Notes [Member]	May 21, 2008 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible debt							$ 73,750,000	$ 73,750,000
Debt instrument interest rate									4.375%
Redemption price, percentage of principal amount							100.00%
Line of credit facility, maximum borrowing capacity	750,000,000
Line of credit adjusted borrowing base	365,000,000
Line of credit facility, current borrowing capacity		530,000,000
Ratio of total debt to EBITDA	2.67					4.00
Current ratio	1.91				1.00
Ratio of senior debt to EBITDA	0					2.5
Ratio of EBITDA to interest expense	4.78				2.5
Line of credit facility amount outstanding			0	0
Letters of credit outstanding amount			$ 900,000