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Long-Term Debt (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended	12 Months Ended
Long-Term Debt (Details) - USD ($) $ in Thousands	Jul. 05, 2016	Jun. 30, 2016	Aug. 31, 2011	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jun. 01, 2016	Apr. 05, 2013
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					$ 3,052	$ 4,316
Long-term Debt					421,948	420,684
Long-term Debt, Excluding Current Maturities					$ 421,948	420,684
Line of Credit Assumed				$ 105,000
3.75 % senior unsecured notes due September 1, 2016
Debt Instrument [Line Items]
Senior unsecured notes, issued amount			$ 250,000
Senior unsecured notes, interest rate			3.75%
Senior unsecured notes, maturity date			Sep. 01, 2016
Term of agreement					5 years
Three Point One Two Five Percent Senior Unsecured Notes Due June Fifteenth Twenty Twenty Five [Domain]
Debt Instrument [Line Items]
Senior unsecured notes, issued amount							$ 425,000
Senior unsecured notes, interest rate				3.125%
Senior unsecured notes, proceeds				$ 421,000
Term of agreement		5 years
Term loan facility [Member]
Debt Instrument [Line Items]
Senior unsecured notes, issued amount								$ 150,000
Unsecured notes
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 425,000	$ 425,000
Unsecured notes \| 3.75 % senior unsecured notes due September 1, 2016
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt	$ 1,300