5. Notes Payable and Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2014	Dec. 31, 2010
Debt Instrument [Line Items]
Debt and Capital Lease Obligations	$ 434,500	$ 434,200
Long-term Debt and Capital Lease Obligations, Maturities, Repayments of Principal Remainder of Fiscal Year	300	300
Long-term Debt and Capital Lease Obligations	434,186	433,883
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	300
Long-term Debt, Maturities, Repayments of Principal in Year Two	300
Long-term Debt, Maturities, Repayments of Principal in Year Three	200
Long-term Debt, Maturities, Repayments of Principal in Year Four	425,300
Long-term Debt, Maturities, Repayments of Principal in Year Five	300
Long-term Debt, Maturities, Repayments of Principal after Year Five	10,600
Line of Credit Facility, Remaining Borrowing Capacity	205,700
Letters of Credit Outstanding, Amount	5,400
Performance Guarantee [Member]
Maturities of Long-term Debt [Abstract]
Letters of Credit Outstanding, Amount	5,400
Unsecured Debt [Member] | Senior Notes 4.875% due 2020 [Member]
Debt Instrument [Line Items]
Unsecured Debt	425,000	425,000
Long-term debt	422,500	421,900		$ 422,600
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	2,500	3,100
Debt Instrument, Interest Rate, Effective Percentage				4.946%
Face amount of debt sold				$ 425,000
Debt Instrument, Interest Rate, Stated Percentage				4.875%
Capital Lease Obligations [Member] | Capital Leases and Other Debt [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities	12,000	$ 12,300
Line of Credit [Member]
Maturities of Long-term Debt [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity			$ 200,000
Line of Credit Facility, Amount Outstanding	$ 0
Line of Credit Facility, Interest Rate at Period End	2.25%
Standby Letters of Credit [Member]
Maturities of Long-term Debt [Abstract]
Letters of Credit Outstanding, Amount	$ 800