Banking Facilities and Debt (Details) $ in Thousands		4 Months Ended	12 Months Ended
	May 07, 2015 USD ($)	May 06, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 27, 2016 USD ($)	Oct. 26, 2016 USD ($)
Banking facilities and other debt
Repayment of term loans				$ 16,667	$ 5,000
Summary of outstanding debt
Total Debt			$ 0	0
Interest rate swaps
Interest rate hedges
Quarterly settlement payment pursuant to hedges			$ 191	$ 920
Minimum | LIBOR
Banking facilities and other debt
Interest rate margin (as a percent)		1.75%
Maximum
Banking facilities and other debt
Pro forma Cash Flow Leverage Ratio to be maintained to purchase, redeem or otherwise acquire shares of common stock			3.00
Cash flow leverage ratio	3.50	3.25
New Revolving Facility
Banking facilities and other debt
Maximum borrowing capacity	$ 75,000
Accordion feature period	4 years
Maximum borrowing capacity accordion feature	$ 50,000
New Revolving Facility | Minimum
Banking facilities and other debt
Commitment fee (as a percent)	0.20%	0.25%
New Revolving Facility | Minimum | LIBOR
Banking facilities and other debt
Interest rate margin (as a percent)	1.00%	1.75%
New Revolving Facility | Minimum | Lender's prime rate
Banking facilities and other debt
Interest rate margin (as a percent)	0.00%
New Revolving Facility | Maximum
Banking facilities and other debt
Commitment fee (as a percent)	0.35%	0.40%
Interest rate margin (as a percent)	2.00%
New Revolving Facility | Maximum | LIBOR
Banking facilities and other debt
Interest rate margin (as a percent)		2.75%
New Revolving Facility | Maximum | Lender's prime rate
Banking facilities and other debt
Interest rate margin (as a percent)	1.00%
Letter of Credit
Banking facilities and other debt
Maximum borrowing capacity						$ 10,000	$ 5,000
Term Loan And Draw Term Loan
Banking facilities and other debt
Repayment of term loans	$ 15,400
Payment for repurchase of hedges	$ 500
Term Loan
Banking facilities and other debt
Term		10 years
Face amount of term loan		$ 40,000
Quarterly principal payments		833
Final principal payment		$ 7,500
Term Loan | LIBOR | Interest rate swaps
Interest rate hedges
LIBOR (as a percent)		4.695%
Current interest rate (as a percent)		6.445%
Draw Term Loan
Banking facilities and other debt
Term		10 years
Face amount of term loan		$ 20,000
Quarterly principal payments		417
Final principal payment		$ 5,400
75% of the outstanding balance of the Draw Term Loan | Interest rate swaps
Interest rate hedges
Percentage of outstanding balance of debt hedged		75.00%
75% of the outstanding balance of the Draw Term Loan | LIBOR | Interest rate swaps
Interest rate hedges
LIBOR (as a percent)		4.875%
Current interest rate (as a percent)		6.625%
25% of the outstanding balance of the Draw Term Loan | Interest rate swaps
Interest rate hedges
Percentage of outstanding balance of debt hedged		25.00%
25% of the outstanding balance of the Draw Term Loan | LIBOR | Interest rate swaps
Interest rate hedges
LIBOR (as a percent)		5.50%
Current interest rate (as a percent)		7.25%
Revolving Facility
Banking facilities and other debt
Maximum borrowing capacity		$ 30,000
Letters of credit outstanding			$ 6,381,000