5. Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2010
Debt Instrument [Line Items]
Debt and Capital Lease Obligations	$ 440,500	$ 435,000
Less Current Maturities	(1,700)	(400)
Long-term debt, net of current maturities	438,776	434,581
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	$ 1,600	1,900
Debt Instrument, Interest Rate, Stated Percentage			4.875%
Debt Instrument, Redemption Price, Percentage	100.00%
Long-term debt	$ 423,400	423,100	$ 422,600
Debt Instrument, Interest Rate, Effective Percentage			4.946%
Face amount of debt sold			$ 425,000
Capital Lease Obligations [Member] | Capital Leases and Other Debt [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities	17,100	$ 11,900
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	3.46%
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 500