Significant Accounting Policies - Convertible Senior Notes (Narrative) (Details) - Convertible Senior Notes - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 115,000,000
5.00% Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Stated interest rate	5.00%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant