Long-Term Debt and Loan Agreements - Additional Information (Details)	1 Months Ended	3 Months Ended				12 Months Ended
	Oct. 31, 2017 USD ($) NoteHolder	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	May 30, 2014 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit		$ 200,000,000				$ 200,000,000
Letters of credit		4,400,000				4,400,000
Long-term Debt		152,632,000				152,632,000	$ 190,686,000
Loss on extinguishment of debt		(1,900,000)				1,888,000	0	$ 0
Unamortized deferred financing costs		1,596,000				1,596,000	1,164,000
Interest Expense [Member]
Debt Instrument [Line Items]
Amortization expense of deferred financing costs						508,000	466,000	$ 465,000
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Face value of debt instrument with offer to be repurchased			$ 100,000,000
Debt instrument repurchase amount	$ 22,000,000
Outstanding principal balance of notes purchased	22,000,000
Debt instrument repurchase amount including make-whole premium	23,800,000
Debt instrument repurchase make-whole premium amount	$ 1,800,000
Debt instrument repurchase date	Oct. 31, 2017
Number of note holder accepted the offer | NoteHolder	1
Loss on extinguishment of debt		1,900,000
Unamortized deferred financing costs		100,000				100,000
Senior Unsecured Note [Member]
Debt Instrument [Line Items]
Long-term Debt		78,000,000				78,000,000
4.67% Senior Unsecured Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 40,000,000				$ 40,000,000	40,000,000
Interest rate		4.67%				4.67%
5.25% Senior Unsecured Notes due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 11,000,000				$ 11,000,000	11,000,000
Interest rate		5.25%				5.25%
5.45% Senior Unsecured Notes due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 12,000,000				$ 12,000,000	$ 20,000,000
Interest rate		5.45%				5.45%
Loan Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit									$ 300,000,000
Loan maturity period						2018-12
Maximum leverage ratio		3.25%	3.50%	3.75%	3.75%
Remaining amount available under the line of credit		$ 121,000,000				$ 121,000,000
Interest rate during period						4.94%	4.69%	4.59%
Loan Agreement [Member] | Extended Maturity [Member]
Debt Instrument [Line Items]
Loan maturity period						2022-03