Debt (Narrative) (Details)		1 Months Ended			9 Months Ended		12 Months Ended
	Mar. 30, 2016 USD ($) debt_extension	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Mar. 27, 2017 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability		$ 67,125,000			$ 67,125,000		$ 67,125,000
Medium term notes, matured		$ 75,000,000
Debt interest rate during period		5.50%
Proceeds from issuance of debt					0	$ 249,999,000
Debt instruments collateral value		$ 700,000,000			700,000,000		700,000,000
Unsecured notes payable under credit facilities		245,000,000			125,000,000		245,000,000
Obligations under capital leases		21,000,000			21,000,000		21,000,000
Net premium/(discount) on debt					(5,600,000)
Deferred finance costs, net					(9,300,000)
Non-cash debt					4,300,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility	$ 500,000,000		$ 500,000,000
Number of credit facility 6-month extensions | debt_extension	2
Line of credit facility, extension period	6 months
Bids amount (up to)					250,000,000
Maximum increase in credit facility amount (up to)					850,000,000
Unsecured notes payable under credit facilities		245,000,000			$ 125,000,000		$ 245,000,000
Facility Fee - basis points					0.15%		0.15%
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities		$ 0			$ 0		$ 0
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio					1.4
Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility								$ 10,000,000
Fixed interest rate loan period (in days)					P30D
Facility Fee - basis points					0.10%		0.10%
Unused Facility Fee - basis points					0.05%		0.10%
Three Point Two Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt payable			$ 250,000,000
Debt stated interest rate			3.25%
Debt issued discount rate			99.16%
Debt effective percentage			3.35%
Proceeds from issuance of debt			$ 246,300,000
Seven Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt payable				$ 90,000,000
Debt stated interest rate				7.50%
Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt stated interest rate				4.50%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing Margin - basis points					0.90%		0.90%
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%
Interest Expense [Member] | Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				$ 2,000,000