Debt with Commercial Bank (Narrative) (Details)					3 Months Ended	12 Months Ended
	Feb. 21, 2018 USD ($) Companies	Nov. 03, 2017 USD ($)	Jun. 17, 2016 USD ($) Companies	Aug. 05, 2014	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 14, 2015 USD ($)
Line of Credit Facility [Line Items]
Proceeds from Lines of Credit					$ 20,000,000
Deferred Costs, Current					449,000	$ 449,000	$ 136,000
Deferred Costs, Noncurrent					1,748,000	1,748,000	$ 298,000
Secured Syndicated Credit Facility, Fifth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion		$ 175,000,000
Proceeds from Bank Debt		20,000,000
Line of Credit Facility, Remaining Borrowing Capacity		30,000,000
Secured Syndicated Credit Facility, Fifth Amendment [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Credit agreement, maximum borrowing capacity	$ 450,000,000
Secured Syndicated Credit Facility, Sixth Amendment [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 650,000,000
Debt Instrument, Term				5 years
Number of Participating Banks | Companies	10
Deferred Costs, Credit Card Origination Costs, Amount					5,710,000	5,710,000
Secured Syndicated Credit Facility, Sixth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 250,000,000
Line of Credit Facility, Term	5 years
Debt Instrument, Periodic Payment, Principal					0
Loans Payable					250,000,000	250,000,000
Loans Payable, Current					12,500,000	$ 12,500,000
Line of Credit Facility, Short Term						12 months
Line of Credit Facility, Periodic Payment, Principal					3,130,000
Short-term Debt					12,500,000	$ 12,500,000
Long-term Debt, Gross					237,500,000	237,500,000
Deferred Costs, Credit Card Origination Costs, Amount					$ 2,200,000	$ 2,200,000
Line of credit, interest rate at period end					3.88%	3.88%
Deferred Costs, Current					$ 449,000	$ 449,000
Deferred Costs, Noncurrent					1,748,000	1,748,000
Secured Syndicated Credit Facility, Sixth Amendment [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 400,000,000
Line of Credit Facility, Term	5 years
Deferred Costs, Credit Card Origination Costs, Amount					$ 3,510,000	$ 3,510,000
Line of credit, interest rate at period end					3.88%	3.88%
Secured Syndicated Credit Facility, Sixth Amendment [Member] | Revolving Credit Facility [Member] | Citi Bank [Member]
Line of Credit Facility [Line Items]
Credit agreement, average amount outstanding during period					$ 224,200,000
Credit agreement, maximum amount outstanding during period					274,500,000
Secured Syndicated Credit Facility, Second Amendment [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 400,000,000
Credit agreement, maximum borrowing capacity			$ 500,000,000
Number of Participating Banks | Companies			11
Newly Added Number of Participating Banks | Companies			7
Secured Syndicated Credit Facility, Second Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 125,000,000
Line of Credit Facility, Term			5 years
Line of Credit Facility, Periodic Payment, Principal					3,130,000
Secured Syndicated Credit Facility, Second Amendment [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 275,000,000
Line of Credit Facility, Term			5 years
Secured Syndicated Credit Facility [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Credit agreement, amount outstanding					$ 173,694,000	$ 173,694,000
Secured Syndicated Credit Facility, First Amendment [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Credit agreement, maximum borrowing capacity								$ 240,000,000
Minimum [Member] | Secured Syndicated Credit Facility, Fifth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion		50,000,000
Maximum [Member] | Secured Syndicated Credit Facility, Fifth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion		$ 100,000,000
Maximum [Member] | Secured Syndicated Credit Facility, Sixth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion	$ 150,000,000