Notes Payable - 2.25% Convertible Senior Notes due 2022 - Balances (Details) - 2.25% Convertible Senior Notes due in 2022 - Convertible Senior Notes - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2015
Liability component:
Aggregate principal amount of notes issued	$ 335,699	$ 335,699	$ 335,700
Less: unamortized debt discount	(72,980)	(80,530)
Less: unamortized debt issuance costs	(5,513)	(5,976)
Net carrying amount	257,206	249,193
Equity component	$ 114,199	$ 114,199

The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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Face (par) amount of debt instrument at time of issuance.

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Amount, after accumulated amortization, of debt discount.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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