Debt (Narrative) (Details) $ in Thousands		3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 30, 2016 USD ($) debt_extension	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) derivative_contract	Mar. 27, 2019 USD ($)	Jan. 01, 2019 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability		$ 60,900	$ 60,900		$ 60,900
Principal payments of debt			3,173	$ 253,955
Gain (loss) on extinguishment of debt					400
Debt instruments collateral value		600,000	600,000		$ 600,000
Finance lease obligation		21,851	21,851				$ 21,900
Net premium/(discount) on debt		(4,300)	(4,300)
Deferred finance costs, net		(6,100)	(6,100)
Non-cash debt		1,600	1,600
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	250,000
Maximum increase in credit facility amount (up to)		$ 850,000	$ 850,000
Commitment fee percentage			0.15%		0.15%
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio		1.4	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
Commitment fee percentage			0.10%		0.10%
Unused capacity, commitment fee percentage			0.05%		0.05%
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]
Basis spread on variable rate			0.90%		0.90%
Thirty-Day LIBOR [Member] | Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Basis spread on variable rate			1.25%		1.25%
Financial Standby Letter of Credit [Member]
Debt Instrument [Line Items]
Securities pledged as collateral		$ 5,300	$ 5,300		$ 5,200
Guarantor obligations, maximum exposure, undiscounted		$ 5,000	$ 5,000		$ 5,000
Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Debt Instrument [Line Items]
Number of interest rate derivatives terminated | derivative_contract					3
Notional amount of interest rate fair value hedge derivatives					$ 200,000
Gain due to the probability that the related hedged forecasted transactions would no longer occur					$ 3,400