Long-Term Debt and Loan Agreements - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
	Oct. 22, 2013	Jan. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 30, 2014	Dec. 13, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Letters of credit			$ 4,300,000
Long-term Debt			193,512,000	$ 237,109,000
Proceeds from issuance of long-term debt			0	89,000,000	$ 11,000,000
Long-term debt, excluding unamortized deferred financing costs			193,006,000	236,429,000
Unamortized debt issuance expense			506,000	680,000
Senior Unsecured Note [Member]
Debt Instrument [Line Items]
Long-term Debt			100,000,000
4.67% Senior Unsecured Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt			$ 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
Proceeds from issuance of long-term debt	$ 11,000,000
Interest rate	5.25%		5.25%
Senior Unsecured Notes Excluding 5.25% Senior Unsecured Notes due in 2024 [Member][Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt		$ 89,000,000
5.45% Senior Unsecured Notes due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt			$ 20,000,000	$ 20,000,000
Interest rate	5.45%		5.45%
Loan Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit						$ 300,000,000	$ 200,000,000	$ 180,000,000
Remaining amount available under the line of credit			$ 202,200,000
Interest rate during period			4.59%	4.00%
Long-term Debt			$ 93,512,000	$ 137,109,000