Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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The fair value of line of credit assumed in noncash investing or financing activities.

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The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

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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, 2017	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2016	Apr. 05, 2013
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					$ 4,316	$ 5,579
Long-term Debt					420,684	516,921
Current portion, long-term debt					0	15,000
Long-term Debt, Excluding Current Maturities					420,684	501,921
Line of Credit Assumed				$ 105,000
Credit Agreement
Debt Instrument [Line Items]
Long-term Debt, Gross					$ 0	97,500
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