Debt (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended		1 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Oct. 31, 2013	Mar. 31, 2013
Debt Instrument [Line Items]
Debt service guaranty liability		$ 72,105,000	$ 73,740,000
Medium term notes, matured		315,000,000	173,600,000
Debt interest rate during period		5.20%	5.40%
Debt payable		1,656,083,000	2,205,104,000
Write-off of debt costs	1,200,000	1,200,000
Debt instruments collateral value		1,000,000,000	1,200,000,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		500,000,000		500,000,000	500,000,000
Credit facility effective date		Apr. 18, 2013
Credit facility expiration date		Apr. 18, 2017
Credit facility description of extensions		provides for two consecutive six-month extensions upon our request
Number of credit facility 6-month extensions		2
Borrowing margin over LIBOR, basis points		115
Facility fees, basis points		20
Bids amount		250,000,000
Maximum increase in credit facility amount		700,000,000
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		99,000,000
Credit facility effective date		May 01, 2010
Fixed interest rate loan period (in days)		30 days
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio		1.4
Debt maturity date		Jan. 01, 2040
8.1% Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes		100,000,000
Debt stated interest rate	8.10%	8.10%
Debt redemption price, percentage		100.00%
Debt maturity date		Sep. 15, 2019
6% Fixed Rate Note [Member]
Debt Instrument [Line Items]
Debt payable			100,000,000
Debt stated interest rate			6.00%
4.45% Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes				250,000,000
Debt stated interest rate				4.45%
Debt maturity date				Jan. 15, 2024
Debt issued at discount				99.58%
Debt effective interest rate				4.50%
Proceeds from issuance of debt				247,300,000
3.5% Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes					300,000,000
Debt stated interest rate					3.50%
Debt maturity date					Apr. 15, 2023
Debt issued at discount					99.53%
Debt effective interest rate					3.56%
Proceeds from issuance of debt					296,600,000
6.75% Series D Preferred Shares [Member]
Debt Instrument [Line Items]
Preferred Shares of beneficial interest, redemption value paid					$ 75,000,000
Preferred shares of beneficial interest, dividend percentage					6.75%