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 ($) Companies	Aug. 05, 2014	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Nov. 27, 2018 USD ($)	Apr. 09, 2018 USD ($)	Oct. 14, 2015 USD ($)
Line of Credit Facility [Line Items]
Proceeds from Lines of Credit					$ 13,500,000
Deferred Costs, Current					575,000	$ 575,000	$ 575,000
Deferred Costs, Noncurrent					$ 1,666,000	1,811,000	$ 1,666,000
Secured Syndicated Credit Facility, Eighth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity								$ 301,250,000.00
Line of Credit Facility, Periodic Payment, Principal						$ 3,770,000
Secured Syndicated Credit Facility, Eighth Amendment [Member] | Revolving Credit Facility [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity								450,000,000
Line of credit, interest rate at period end					5.00%		5.00%
Secured Syndicated Credit Facility, Seventh Amendment [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Permitted indebtedness in the form of unsecured convertible notes									$ 300,000,000
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]
Permitted indebtedness in the form of unsecured convertible notes	250,000,000
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					$ 5,500,000		$ 5,500,000
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					4,000,000
Loans Payable					287,500,000		287,500,000
Loans Payable, Current					15,100,000		$ 15,100,000
Line of Credit Facility, Short Term							12 months
Line of Credit Facility, Periodic Payment, Principal					3,130,000
Short-term Debt					15,100,000		$ 15,100,000
Long-term Debt, Gross					272,400,000		272,400,000
Deferred Costs, Credit Card Origination Costs, Amount					2,200,000		2,200,000
Deferred Costs, Current					575,000		575,000
Deferred Costs, Noncurrent					1,666,000		1,666,000
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					$ 3,300,000		$ 3,300,000
Line of credit, interest rate at period end					5.00%		5.00%
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					$ 434,400,000
Credit agreement, maximum amount outstanding during period					438,000,000
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					$ 438,037,000		$ 438,037,000
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, Eighth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion								101,250,000
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, Eighth Amendment [Member] | Secured Term Loan [Member] | Regions Bank [Member]
Line of Credit Facility [Line Items]
Line of Credit, Accordion								$ 150,000,000
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