Debt (Narrative) (Details)				1 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 30, 2016 USD ($) extensions	Jun. 30, 2016 USD ($)	May 31, 2015 USD ($)	Mar. 31, 2015	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) extensions	Jun. 01, 2016	May 31, 2016 USD ($)	Mar. 27, 2016 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2014
Debt Instrument [Line Items]
Debt service guaranty liability	$ 69,835,000			$ 69,835,000			$ 69,835,000		$ 69,835,000
Proceeds from issuance of debt							2,111,000	$ 448,083,000
Medium term notes, matured									$ 90,000,000
Debt interest rate during period									5.40%
Debt instruments collateral value	700,000,000			700,000,000			700,000,000		$ 800,000,000
Unsecured notes payable under credit facilities	98,000,000			98,000,000			98,000,000		149,500,000
Obligations under capital leases	21,000,000			21,000,000			21,000,000		21,000,000
Net premium/(discount) on debt	(4,300,000)			(4,300,000)			(4,300,000)
Deferred finance costs, net	(9,400,000)			(9,400,000)			(9,400,000)
Non-cash debt	$ 5,600,000			5,600,000			5,600,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		$ 500,000,000	$ 500,000,000		$ 500,000,000				$ 500,000,000				$ 500,000,000
Number of credit facility 6-month extensions | extensions			2						2
Line of credit facility, extension period			6 months						6 months
Facility fees, basis points	15.00%								15.00%
Bids amount (up to)	$ 250,000,000			250,000,000			250,000,000		$ 250,000,000
Maximum increase in credit facility amount (up to)	850,000,000			850,000,000			850,000,000		700,000,000
Unsecured notes payable under credit facilities	95,000,000			95,000,000			95,000,000		140,000,000
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities	$ 3,000,000			$ 3,000,000			$ 3,000,000		9,500,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio	1.4			1.4			1.4
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt payable		$ 200,000,000
Debt effective percentage		2.50%
Maximum increase in term loan amount		$ 100,000,000
Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility								$ 20,000,000				$ 10,000,000
Facility fees, basis points		10.00%
Fixed interest rate loan period (in days)		30 days
Unused facility fees, basis points		10.00%
Seven Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt payable											$ 90,000,000
Debt stated interest rate											7.49%
Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt stated interest rate										4.45%
Three Point Eight Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt payable					$ 250,000,000
Debt stated interest rate					3.85%
Debt issued discount rate					99.23%
Debt effective percentage					3.94%
Proceeds from issuance of debt					$ 246,500,000
Seven Point Four Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt payable									$ 66,000,000
Debt stated interest rate														7.40%
Three Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt stated interest rate									3.50%
Thirty-Day LIBOR [Member] | Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points		125.00%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points	90.00%								105.00%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points						97.50%
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
Interest Expense [Member] | Three Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt							$ (6,100,000)