Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Indebtedness (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2011	Jun. 30, 2011
Debt Instrument [Line Items]
Total Senior Convertible Notes	$ 669,398	$ 372,920
2.75% Senior Convertible Notes [Member] \| Convertible Notes due 2017 [Member]
Debt Instrument [Line Items]
Principal amount	125,732	402,500		$ 402,500
Unamortized debt discount	(6,818)	(25,958)	$ (88,900)
Unamortized debt issuance costs	(957)	(3,622)
Total Senior Convertible Notes	117,957	$ 372,920
2.25% Senior Convertible Notes [Member] \| Convertible Notes due 2021 [Member]
Debt Instrument [Line Items]
Principal amount	650,000
Unamortized debt discount	(84,144)
Unamortized debt issuance costs	(14,415)
Total Senior Convertible Notes	$ 551,441