6. Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2010
Debt Instrument [Line Items]
Debt and Lease Obligation	$ 438,700	$ 439,800
Other Long-term Debt, Current	426,500	426,200
Long-term debt, net of current maturities	12,233	13,579
Unsecured Debt [Member] | Senior Notes 4.875% due 2020 [Member]
Debt Instrument [Line Items]
Unsecured Debt	425,000	425,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 300	600
Debt Instrument, Interest Rate, Stated Percentage			4.875%
Debt Instrument, Redemption Price, Percentage	100.00%
Long-term debt	$ 424,700	424,400	$ 422,600
Debt Instrument, Interest Rate, Effective Percentage			4.946%
Face amount of debt sold			$ 425,000
Finance Lease Obligations [Member] | Finance Leases and Other Debt [Member]
Debt Instrument [Line Items]
Other Long-term Debt	14,000	$ 15,400
Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	200,000
Outstanding borrowings	$ 0
Line of Credit Facility, Interest Rate at Period End	1.425%
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 200