Debt with Commercial Bank (Narrative) (Details)					3 Months Ended		6 Months Ended	12 Months Ended
	Feb. 21, 2018 USD ($) Companies	Nov. 03, 2017 USD ($)	Jun. 17, 2016 USD ($) Companies	Aug. 05, 2014	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	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		$ 575,000	$ 575,000
Deferred Costs, Noncurrent					$ 1,523,000	1,811,000	$ 1,523,000		$ 1,523,000	1,811,000
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
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%		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					$ 5,200,000	5,900,000	$ 5,200,000		$ 5,200,000	5,900,000
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							7,530,000
Loans Payable					283,700,000	291,200,000	283,700,000		283,700,000	291,200,000
Loans Payable, Current					16,900,000	15,100,000	16,900,000		$ 16,900,000	15,100,000
Line of Credit Facility, Short Term								12 months	12 months
Line of Credit Facility, Periodic Payment, Principal					3,130,000
Short-term Debt					16,900,000	15,100,000	16,900,000		$ 16,900,000	15,100,000
Long-term Debt, Gross					266,800,000	276,200,000	266,800,000		266,800,000	276,200,000
Deferred Costs, Credit Card Origination Costs, Amount					2,100,000	2,400,000	2,100,000		2,100,000	2,400,000
Deferred Costs, Current					575,000	575,000	575,000		575,000	575,000
Deferred Costs, Noncurrent					1,500,000	1,800,000	1,500,000		1,500,000	1,800,000
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					$ 3,100,000	$ 3,500,000	$ 3,100,000		$ 3,100,000	$ 3,500,000
Line of credit, interest rate at period end					5.00%	4.88%	5.00%		5.00%	4.88%
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							$ 436,200,000			$ 318,900,000
Credit agreement, maximum amount outstanding during period							438,000,000.0			424,500,000
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					$ 438,000,000.0	$ 424,500,000	$ 438,000,000.0		$ 438,000,000.0	$ 424,500,000
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