Debt (Narrative) (Details) $ in Thousands		6 Months Ended		12 Months Ended
	Mar. 30, 2016 USD ($) debt_extension	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 27, 2018 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability		$ 64,145		$ 64,145
Principal payments of debt		253,955	$ 21,308
Gain (loss) on extinguishment of debt		400
Debt instruments collateral value		600,000		700,000
Obligations under capital leases		21,000		$ 21,000
Net premium/(discount) on debt		(5,000)
Deferred finance costs, net		(7,600)
Non-cash debt		2,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility	$ 500,000
Number of credit facility 6-month extensions | debt_extension	2
Line of credit facility, extension period	6 months
Bids amount (up to)		250,000
Maximum increase in credit facility amount (up to)		$ 850,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio		1.4
Par Value Debt [Member]
Debt Instrument [Line Items]
Principal payments of debt		$ 51,000
Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility					$ 10,000
Fixed interest rate loan period (in days)		30 days		30 days
Unsecured Variable-Rate Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, repurchased face amount		$ 200,000
Debt instrument, interest rate, effective percentage		2.50%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing Margin - basis points		0.90%		0.90%
Facility Fee - basis points		0.15%		0.15%
London Interbank Offered Rate (LIBOR) [Member] | Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Borrowing Margin - basis points		1.25%		1.25%
Facility Fee - basis points		0.10%		0.10%
Unused Facility Fee - basis points		0.05%		0.05%
Financial Standby Letter of Credit [Member]
Debt Instrument [Line Items]
Securities pledged as collateral		$ 4,500
Guarantor obligations, maximum exposure, undiscounted		$ 4,300