Debt (Narrative) (Details)		1 Months Ended			3 Months Ended	9 Months Ended		12 Months Ended
	Apr. 18, 2013 USD ($) extensions	May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	May. 31, 2010 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability						$ 72,105,000		$ 72,105,000
Debt stated interest rate						3.50%		7.40%
Proceeds from issuance of debt						$ 448,083,000	$ 4,500,000
Medium term notes, matured						$ 90,000,000		$ 315,000,000
Debt interest rate during period						5.40%		5.20%
Secured debt						$ 66,000,000
Write off of debt costs					$ 1,200,000
Debt instruments collateral value						$ 800,000,000		$ 1,000,000,000
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
Number of credit facility 6-month extensions | extensions	2
Line of credit facility, extension period	6 months
Facility fees, basis points						0.20%
Bids amount (up to)						$ 250,000,000
Maximum increase in credit facility amount (up to)						$ 700,000,000
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility				$ 99,000,000
Fixed interest rate loan period (in days)				30 days
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio						1.4
Term Loan [Member]
Debt Instrument [Line Items]
Debt payable			$ 200,000,000
Debt effective percentage			2.60%
Maximum increase in term loan amount			$ 100,000,000
8.1% Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt stated interest rate								8.10%
Senior unsecured notes								$ 100,000,000
Debt redemption price, percentage								100.00%
Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility									$ 20,000,000
Facility fees, basis points			0.10%
Fixed interest rate loan period (in days)			30 days
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
Thirty-Day LIBOR [Member] | Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points			1.25%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points						1.15%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points			1.15%
Interest Expense [Member]
Debt Instrument [Line Items]
Payments of debt extinguishment costs						$ 6,100,000