Long Term Debt (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 04, 2016
Debt Instrument [Line Items]
Loss on early extinguishment of debt	$ 300,000	$ 0	$ 254,000	$ 0
Long-term debt outstanding		$ 486,443,000	498,490,000
Interest rate at period end (percent)		3.57%
Debt Instrument, Financing Costs Expensed	$ 2,700,000		2,700,000
Revolving Line of Credit [Member]
Debt Instrument [Line Items]
Long-term debt outstanding		$ 327,000,000	329,000,000
Line of credit facility, available borrowing capacity		194,900,000
Revolving Line of Credit [Member] | Amended and Restated Senior Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 525,000,000.0
Letters of Credit [Member]
Debt Instrument [Line Items]
Letters of credit outstanding		3,100,000
Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		157,500,000
Long-term debt outstanding		157,033,000	165,628,000
Term Loan Facility [Member] | Amended and Restated Senior Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 175,000,000.0
Mortgages Notes [Member]
Debt Instrument [Line Items]
Long-term debt outstanding		$ 2,410,000	$ 3,862,000
Debt interest rate (percent)		8.25%
London Interbank Offered Rate (LIBOR) [Member] | Long-term Debt [Member] | Amended and Restated Senior Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (percent)		2.00%
Federal Funds Effective Swap Rate [Member] | Long-term Debt [Member] | Amended and Restated Senior Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (percent)		0.50%
Eurodollar [Member] | Long-term Debt [Member] | Amended and Restated Senior Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (percent)		1.00%