Senior Convertible Notes (Details Textual) (USD $)	12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2013 2.75% Senior Convertible Notes due 2017 [Member]	Dec. 31, 2012 2.75% Senior Convertible Notes due 2017 [Member]	Dec. 31, 2011 2.75% Senior Convertible Notes due 2017 [Member]	Dec. 31, 2011 2.25% Senior Convertible Notes Due 2013 [Member]	Dec. 31, 2008 2.25% Senior Convertible Notes Due 2013 [Member]	Dec. 31, 2013 Minimum [Member] 2.75% Senior Convertible Notes due 2017 [Member]	Dec. 31, 2013 Maximum [Member] 2.75% Senior Convertible Notes due 2017 [Member]
Senior Convertible Notes (Textual) [Abstract]
Interest rate on convertible notes							2.75%		2.25%
Principal amount of senior convertible notes					$ 402,500,000	$ 402,500,000	$ 402,500,000		$ 230,000,000
Additional purchase of unsecured senior convertible notes							52,500,000
Net proceeds of unsecured senior convertible notes							359,200,000		208,400,000
Fair value of outstanding notes					439,300,000	361,300,000
Consecutive trading days immediately preceding calendar quarter										20 days	30 days
Percentage of conversion price										130.00%	98.00%
Business day period					5 days
Number of common shares authorized	120,000,000	120,000,000	70,000,000	70,000,000
Repurchase of Senior Convertible Notes	0	0	(154,164,000)					155,700,000
Aggregate repurchase price of senior convertible notes including accrued interest								155,500,000
Debt Instrument accrued interest rate								99.00%
Loss on extinguishment of debt	0	0	(332,000)					300,000
Repayments of Long-term Debt	(74,311,000)	0	0
Outstanding debt instrument	0	74,311,000
Initial conversion rate adjustment Shares							23.7344
Base principal amount per note							1,000
Initial conversion price of convertible notes							$ 42.13
Fair value of debt conversion cost							88,900,000
Debt discount, interest rate							8.00%
Debt Instrument, convertible carrying amount		49,300,000			49,300,000
Contractual Coupon Interest Expense					11,100,000	11,100,000
Amortization of Debt Discount (Premium)	13,656,000	12,697,000	6,108,000		13,700,000	12,700,000
Number of common stock to be purchased							9,553,096
Initial stock price at which shares to be purchased			$ 42.13				$ 42.13
Number of warrants to be acquired							477,654		5,100,000
Initial strike price at which Warrants to be acquired							988.51		49.13
Number of common stock shares preferred stock convertible into							20
Cash proceeds from the sale of warrants	0	0	47,898,000				47,900,000
Cost of hedge transaction							$ 80,100,000
Debt instrument, maturity date							Jul. 01, 2017