Debt (Narrative) (Details)			1 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) extensions	Mar. 30, 2016 USD ($) extensions	Mar. 27, 2016 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2014
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities	$ 180,000,000	$ 180,000,000			$ 180,000,000		$ 149,500,000
Debt service guaranty liability	69,835,000	69,835,000			69,835,000		$ 69,835,000
Debt stated interest rate							3.50%				7.40%
Proceeds from issuance of debt					0	$ 200,000,000
Medium term notes, matured							$ 90,000,000
Debt interest rate during period							5.40%
Secured debt							$ 66,000,000
Debt instruments collateral value	800,000,000	800,000,000			800,000,000		800,000,000
Obligations under capital leases	21,000,000	21,000,000			21,000,000		21,000,000
Fair value of interest rate contracts	2,600,000	2,600,000			2,600,000
Net premium/(discount) on debt	(4,400,000)	(4,400,000)			(4,400,000)
Deferred finance costs, net	(9,500,000)	(9,500,000)			(9,500,000)
Non-cash debt	5,900,000	5,900,000			5,900,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities	180,000,000	$ 180,000,000			180,000,000		140,000,000
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	2
Line of credit facility, extension period		6 months					6 months
Facility fees, basis points		0.00%					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 And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities	$ 0	$ 0			$ 0		$ 9,500,000
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio	1.4	1.4			1.4
Term Loan [Member]
Debt Instrument [Line Items]
Debt payable				$ 200,000,000		$ 200,000,000
Debt effective percentage				2.50%		2.50%
Maximum increase in term loan amount				$ 100,000,000		$ 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		$ 20,000,000			$ 10,000,000
Facility fees, basis points	0.00%
Unused facility fees, basis points	10.00%
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		0.90%					105.00%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points				0.975%
Interest Expense [Member]
Debt Instrument [Line Items]
Payments of debt extinguishment costs					$ 6,100,000