Commercial Bank Financing Facility (Details)				3 Months Ended		6 Months Ended	12 Months Ended
	Feb. 21, 2018 USD ($) Companies	Nov. 03, 2017 USD ($)	Jun. 16, 2016 Companies	Dec. 31, 2018 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 27, 2018 USD ($)	Apr. 09, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 17, 2016 USD ($)	Oct. 14, 2015 USD ($)	Feb. 03, 2015 USD ($)	Aug. 05, 2014 USD ($)
Secured Syndicated Credit Facility, Eighth Amendment | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity								$ 450,000,000
Secured Syndicated Credit Facility, Eighth Amendment | Secured Term Loan | Regions Bank
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, Fifth Amendment | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Credit agreement, maximum borrowing capacity	$ 450,000,000
Secured Syndicated Credit Facility, Fifth Amendment | Secured Term Loan | Regions Bank
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 | Regions Bank
Line of Credit Facility [Line Items]
Debt instrument, face amount														$ 150,000,000
Secured Syndicated Credit Facility | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Expansion														50,000,000
Line of Credit, Accordion												$ 50,000,000		50,000,000
Credit agreement, maximum borrowing capacity														$ 200,000,000
Secured Syndicated Credit Facility | Revolving Credit Facility | Citi Bank
Line of Credit Facility [Line Items]
Credit agreement, amount outstanding				$ 424,500,000			$ 424,500,000			$ 274,500,000
Line of credit, interest rate at period end				4.875%			4.875%
Credit agreement, average amount outstanding during period							$ 318,900,000
Credit agreement, maximum amount outstanding during period							$ 424,500,000
Weighted average interest rate				4.51%			4.51%			4.13%
Secured Syndicated Credit Facility, Second Amendment | Regions Bank
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 | Revolving Credit Facility | Regions Bank
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, Second Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity											$ 125,000,000
Line of Credit Facility, Periodic Payment, Principal					$ 3,130,000
Loans Payable				$ 291,200,000			$ 291,200,000
Loans Payable, Current				15,100,000			15,100,000
Line of Credit Facility, Term			5 years
Short-term Debt				15,100,000			15,100,000
Long-term Debt, Gross				$ 276,200,000			$ 276,200,000
Secured Syndicated Credit Facility, First Amendment | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Expansion													$ 90,000,000
Debt instrument, face amount													190,000,000
Credit agreement, maximum borrowing capacity												240,000,000	$ 240,000,000
Secured Syndicated Credit Facility, First Amendment | Revolving Credit Facility | TD Bank
Line of Credit Facility [Line Items]
Credit agreement, maximum borrowing capacity												$ 25,000,000
Secured Syndicated Credit Facility, Sixth Amendment | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 650,000,000
Number of Participating Banks | Companies	10
Permitted indebtedness in the form of unsecured convertible notes	$ 250,000,000
Secured Syndicated Credit Facility, Sixth Amendment | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 400,000,000
Line of Credit Facility, Term	5 years
Secured Syndicated Credit Facility, Sixth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 250,000,000
Line of Credit Facility, Periodic Payment, Principal						$ 3,130,000
Line of Credit Facility, Term	5 years
Secured Syndicated Credit Facility, Seventh Amendment | Regions Bank
Line of Credit Facility [Line Items]
Permitted indebtedness in the form of unsecured convertible notes									$ 300,000,000
Minimum | Secured Syndicated Credit Facility, Eighth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit, Accordion								101,250,000
Minimum | Secured Syndicated Credit Facility, Fifth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit, Accordion		50,000,000
Maximum | Secured Syndicated Credit Facility, Eighth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit, Accordion								$ 150,000,000
Maximum | Secured Syndicated Credit Facility, Fifth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit, Accordion		$ 100,000,000
Maximum | Secured Syndicated Credit Facility, Sixth Amendment | Secured Term Loan | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit, Accordion	$ 150,000,000