Commercial Bank Financing Facility (Details)		3 Months Ended	12 Months Ended
	Jun. 17, 2016 USD ($) Companies	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 14, 2015 USD ($)	Feb. 03, 2015 USD ($)	Aug. 05, 2014 USD ($)
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			$ 154,030,000	$ 206,500,000
Line of credit, interest rate at period end			2.875%
Credit agreement, average amount outstanding during period			$ 168,000,000
Credit agreement, maximum amount outstanding during period			$ 226,460,000
Weighted average interest rate			2.72%	2.25%
Secured Syndicated Credit Facility, Second Amendment [Member] | 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 [Member] | Revolving Credit Facility | Regions Bank
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 275,000,000
Line of credit, interest rate at period end			2.875%
Line of Credit Facility, Term	5 years
Secured Syndicated Credit Facility, Second Amendment [Member] | Secured Term Loan [Member] | 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			$ 118,800,000
Loans Payable, Current			12,500,000
Line of Credit Facility, Term	5 years
Short-term Debt			12,500,000
Long-term Debt, Gross			$ 106,300,000
Secured Syndicated Credit Facility, First Amendment [Member] | 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 [Member] | Revolving Credit Facility | TD Bank [Member]
Line of Credit Facility [Line Items]
Credit agreement, maximum borrowing capacity					$ 25,000,000