Long-Term Indebtedness (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Unamortized deferred financing fees	$ (1,152)	$ (1,291)
Long-term debt	793,762	738,249
Less current portion	(67,154)	(17,831)
Long-term debt, excluding current maturities	726,608	720,418
Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross	454,706	394,888
Shelf Loan
Debt Instrument [Line Items]
Long-term debt, gross	50,000	50,000
Other
Debt Instrument [Line Items]
Long-term debt, gross	8,958	9,652
Medium-term Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 281,250	$ 285,000

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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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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