Debt (Narrative) (Details)	1 Months Ended			3 Months Ended	12 Months Ended
	May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	May. 31, 2010 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) extensions	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Feb. 28, 2015 USD ($)	Apr. 18, 2013 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability					$ 69,835,000	$ 72,105,000
Debt stated interest rate					3.50%	7.40%
Proceeds from issuance of debt					$ 448,083,000	$ 4,500,000	$ 573,542,000
Medium term notes, matured					$ 90,000,000	$ 315,000,000
Debt interest rate during period					5.40%	5.20%
Secured debt					$ 66,000,000
Debt extinguishment costs				$ 6,100,000
Write-off of debt costs						$ 1,200,000
Debt instruments collateral value					800,000,000	1,000,000,000
Unsecured notes payable under credit facilities					149,500,000	189,000,000
Capital leases					21,000,000	$ 21,000,000
Fair value of interest rate contracts					2,700,000
Net premium (discount) on debt					(4,600,000)
Deferred debt costs					(10,100,000)
Non-cash debt					$ 5,100,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio					1.4
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 200,000,000		$ 200,000,000
Debt instrument, interest rate, effective percentage		2.50%		2.50%
Debt instrument, accordion feature		$ 100,000,000		$ 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%
Three Point Eight Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 250,000,000
Debt stated interest rate	3.85%
Debt instrument, issued at discount	99.23%
Debt instrument, interest rate, effective percentage	3.94%
Proceeds from issuance of debt	$ 246,500,000
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points		0.975%
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
Facility fees, basis points					0.15%
Bids amount (up to)					$ 250,000,000
Increase in credit facility amount (up to)					700,000,000
Unsecured notes payable under credit facilities					$ 140,000,000	$ 189,000,000
Unsecured Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points					1.05%
Line of Credit [Member] | Short-Term Unsecured Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		$ 20,000,000		$ 20,000,000
Facility fees, basis points		0.10%
Fixed interest rate loan period (in days)		30 days
Line of Credit [Member] | Thirty-Day LIBOR [Member] | Short-Term Unsecured Facility [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points		1.25%
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
Unsecured notes payable under credit facilities					$ 9,500,000	$ 0
Interest Expense [Member]
Debt Instrument [Line Items]
Debt extinguishment costs					$ 6,100,000