Organization and Basis of Presentation - Additional Information (Details) - USD ($)			3 Months Ended
	Mar. 18, 2016	Jan. 22, 2016	Mar. 31, 2016	Jun. 30, 2015	Apr. 13, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash			$ 35,806,000			$ 1,272,000
Available for borrowings			1,400,000,000			1,177,500,000
Line of credit, current borrowing capacity			1,400,000,000			1,400,000,000
Proceeds from issuance of common stock net of offering expenses			381,137,000	$ 398,600,000
Gross amount			$ 554,000,000			776,500,000
One-time Termination Benefits
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Restructuring charges incurred as a result of workforce reduction	$ 800,000	$ 3,200,000
Common Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Shares issued through public equity offering			18,170,000	5,700,000
Long-term note agreements
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross amount			$ 554,000,000
Credit facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross amount			$ 0			$ 222,500,000
Subsequent Event | Credit facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Line of credit, current borrowing capacity					$ 1,050,000,000