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Notes Payable (Narrative) (Details) $ in Thousands			9 Months Ended	12 Months Ended
	Mar. 29, 2017 USD ($)	Jul. 02, 2013 USD ($)	Sep. 30, 2017	Dec. 31, 2017 USD ($)	Mar. 08, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 03, 2014 USD ($)
Debt Instrument [Line Items]
Revolving credit facility term		5 years
Unamortized discount and debt issuance costs				$ 3,665		$ 0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Loan maximum borrowing capacity		$ 200,000					$ 250,000
Revolving credit facility term	5 years
Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee on undrawn portion of facility	0.125%
Debt to total capital ratio	0.15
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Commitment fee on undrawn portion of facility	0.225%
Debt to total capital ratio	0.25
Revolving Credit Facility [Member] | LIBOR [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.125%
Debt to total capital ratio	0.15
Revolving Credit Facility [Member] | LIBOR [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.625%
Debt to total capital ratio	0.25
Secured Notes Two [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Loan maximum borrowing capacity	$ 50,000
Secured Notes Two [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee on undrawn portion of facility				0.15%
Unsecured Notes Three [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Unamortized debt issuance expense	$ 200
Unsecured Notes Three [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt to total capital ratio				0.176
Level 2 [Member] | Secured Notes Two [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate			0.40%
Unsecured Debt [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage					4.40%
Debt issuance costs					$ 3,400
Discount percent					99.847%
Interest rate, effective percentage					4.45%
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Two [Member]
Debt Instrument [Line Items]
Unsecured debt				$ 375,000		$ 0
Interest rate, stated percentage			4.40%
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Three [Member] | LIBOR [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate			0.1125%
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Three [Member] | LIBOR [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate			0.1625%

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