Debt (Narrative) (Details) $ in Thousands		1 Months Ended			3 Months Ended	12 Months Ended
	Mar. 30, 2016 USD ($) debt_extension	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 27, 2017 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability		$ 67,125				$ 64,145	$ 67,125
Repayments of medium term notes		$ 75,000
Debt instrument, interest rate during period		5.50%
Proceeds from issuance of debt						0	249,999	$ 448,083
Gain (loss) on debt extinguishment					$ 2,000
Debt instruments collateral value		$ 700,000				700,000	700,000
Capital leases		21,000				21,000	21,000
Net premium (discount) on debt						(5,400)
Deferred debt costs						(8,900)
Non-cash debt						$ 4,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio						1.4
Secured Debt [Member]
Debt Instrument [Line Items]
Deferred debt costs						$ (1,300)
Non-cash debt						4,000
Three Point Two Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 250,000
Debt stated interest rate			3.25%
Debt instrument, issued at discount			99.16%
Debt instrument, interest rate, effective percentage			3.35%
Proceeds from issuance of debt			$ 246,300
Seven Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 90,000					$ 90,000
Debt stated interest rate		7.50%					7.50%
Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt stated interest rate		4.50%					4.50%
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility	$ 500,000		$ 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 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, description of variable rate basis						P30D
Line of credit facility, commitment fee percentage						0.10%	0.10%
Facility fees, basis points						0.05%	0.10%
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%
Interest Expense [Member] | Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on debt extinguishment				$ 2,000