Debt (Narrative) (Details)				1 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Mar. 31, 2016	Mar. 30, 2016 USD ($) extensions	Aug. 31, 2016 USD ($)	Jul. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	May 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Jun. 01, 2016	May 31, 2016 USD ($)	Mar. 27, 2016 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2014
Debt Instrument [Line Items]
Debt, net	$ 2,111,938,000								$ 2,111,938,000
Debt service guaranty liability	69,835,000								69,835,000		$ 69,835,000
Proceeds from issuance of debt									249,999,000	$ 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		$ 800,000,000
Unsecured notes payable under credit facilities	130,000,000								130,000,000		149,500,000
Obligations under capital leases	21,000,000								21,000,000		21,000,000
Net premium/(discount) on debt	(6,300,000)								(6,300,000)
Deferred finance costs, net	(11,000,000)								(11,000,000)
Non-cash debt	$ 5,300,000								5,300,000
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				$ 500,000,000
Number of credit facility 6-month extensions | extensions			2
Line of credit facility, extension period			6 months
Facility fees, basis points	15.00%										15.00%
Bids amount (up to)	$ 250,000,000								250,000,000		$ 250,000,000
Maximum increase in credit facility amount (up to)	850,000,000								850,000,000		700,000,000
Unsecured notes payable under credit facilities	128,000,000								128,000,000		140,000,000
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities	$ 2,000,000								$ 2,000,000		9,500,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio	1.4								1.4
Unsecured Debt [Member]
Debt Instrument [Line Items]
Optional extension term					1 year
Debt, net	$ 0								$ 0
Debt payable					$ 200,000,000			$ 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%
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													7.49%
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)