Debt (Narrative) (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Mar. 30, 2016 USD ($) debt_extension	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 27, 2018 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability			$ 60,900	$ 64,145
Principal payments of debt			257,028	28,723	$ 144,788
Gain (loss) on debt extinguishment		$ 3,800	400
Debt instruments collateral value			600,000	700,000
Unsecured notes payable under credit facilities			5,000	0
Capital leases			21,898	$ 21,000
Net premium (discount) on debt			(4,600)
Deferred debt costs			(6,900)
Non-cash debt			$ 1,800
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
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%
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
Increase in credit facility amount (up to)			$ 850,000
Line of credit facility, commitment fee percentage			0.15%	0.15%
Unsecured notes payable under credit facilities			$ 5,000	$ 0
Unsecured Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points			0.90%	0.90%
Line of Credit [Member] | Short-Term Unsecured Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility						$ 10,000
Debt instrument, term			30 days	30 days
Line of credit facility, commitment fee percentage			0.10%	0.10%
Facility fees, basis points			0.05%	0.05%
Line of Credit [Member] | Thirty-Day LIBOR [Member] | Short-Term Unsecured Facility [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points			1.25%	1.25%
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities			$ 0	$ 0
Financial Standby Letter of Credit [Member]
Debt Instrument [Line Items]
Held-to-maturity, restricted			5,200
Guarantor obligations, maximum exposure, undiscounted			$ 5,000