Convertible Senior Notes (Details) (USD $)	3 Months Ended		9 Months Ended							0 Months Ended	3 Months Ended		9 Months Ended	0 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Conversion upon satisfaction of closing market price condition item	Sep. 30, 2013 Conversion upon satisfaction of closing market price condition Minimum	Sep. 30, 2013 Conversion upon satisfaction of closing market price condition Maximum	Sep. 30, 2013 Conversion upon satisfaction of trading price condition	Sep. 30, 2013 Conversion upon satisfaction of trading price condition Maximum	Feb. 15, 2013 2018 Notes	Sep. 30, 2013 2018 Notes	Jun. 30, 2013 2018 Notes	Sep. 30, 2013 2018 Notes	Jul. 03, 2013 2019 Notes	Sep. 30, 2013 2019 Notes
Convertible Senior Notes
Interest rate (as a percent)											4.55%		4.55%		4.00%
Discount at which debt issued to underwriters (as a percent)										2.05%				2.125%
Gross proceeds			$ 1,037,926,000							$ 587,700,000				$ 450,200,000
Principal Amount										600,000,000	1,060,000,000		1,060,000,000	460,000,000
Effective Rate (as a percent)													6.08%		5.37%
Conversion Rate													35.5981		37.9896
Remaining Period of Amortization													4 years 4 months 24 days		5 years 3 months 18 days
Net unamortized discount											(64,928,000)		(64,928,000)
Total	1,312,750,000		1,312,750,000								995,072,000		995,072,000
Carrying amount of conversion option equity components recorded in additional paid-in capital			48,502,000								48,502,000		48,502,000
Principal amount of notes, basis for conversion											1,000		1,000
Initial exchange price (in dollars per share)											35.5391
Dividend distributions declared per common share (in dollars per share)	$ 0.46	$ 0.44	$ 1.36	$ 1.32							$ 0.46	$ 0.46
Dividend distributions initially declared per common share (in dollars per share)													$ 0.44		$ 0.46
Closing share price (in dollars per share)	$ 23.97		$ 23.97
Conversion price (in dollars per share)											$ 28.09		$ 28.09		$ 26.32
Proceeds from notes attributed to equity component	48,500,000		48,500,000
Aggregate contractual interest expense	11,400,000		21,500,000
Additional non-cash interest expense	$ 3,000,000		$ 5,700,000
Minimum number of conditions to be satisfied for conversion of debt					1
Percentage of conversion price as a basis for debt conversion						130.00%
Minimum trading period as a basis for debt conversion					20
Consecutive trading period as a basis for debt conversion					30 days			5 days
Percentage of conversion price and last reported sales price as a basis for debt conversion									98.00%
Period of average closing market price of common stock as a basis for debt conversion							10 days
Percentage of per share value of distributions that exceeds the market price of the entity's common stock as a basis for debt conversion						10.00%