Common Stock Sale of Common Stock (Details) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Mar. 31, 2015
Class of Stock [Line Items]
Common stock, par value	$ 0.01		$ 0.01
Proceeds from Issuance of Common Stock	$ 855,072,000	$ 202,851,000
Common shares - par value	563,000		$ 402,000
Additional paid-in capital	1,796,664,000		$ 907,382,000
Repayments of Convertible Debt	$ 115,000,000	$ 0
September 2016 Common Stock Issuance [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	9,085,000
Sale of Stock, Price Per Share	$ 61.51
Proceeds from Issuance of Common Stock	$ 558,500,000
Common shares - par value	90,850
Additional paid-in capital	$ 558,400,000
March 2016 Common Stock Issuance
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	5,922,500
Sale of Stock, Price Per Share	$ 50.11
Proceeds from Issuance of Common Stock	$ 296,600,000
Common shares - par value	59,225
Additional paid-in capital	$ 296,500,000
March 2015 Common Stock Issuance
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues		4,002,000
Sale of Stock, Price Per Share				$ 50.73
Proceeds from Issuance of Common Stock		$ 202,900,000
Common shares - par value				$ 40,020
Additional paid-in capital				$ 202,800,000