Debt (Narrative) (Details)			1 Months Ended				3 Months Ended		12 Months Ended
	Mar. 30, 2016 USD ($) extensions	Mar. 27, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	May 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 01, 2016	May 31, 2016 USD ($)	Mar. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Debt service guaranty liability									$ 67,125,000	$ 69,835,000
Proceeds from issuance of debt									249,999,000	448,083,000	$ 4,500,000
Debt, net									2,034,998,000
Gain (loss) on debt extinguishment							$ 2,000,000	$ (6,100,000)
Repayments of medium term notes									$ 75,000,000	$ 90,000,000
Debt instrument, interest rate during period									5.50%	5.40%
Debt instruments collateral value									$ 700,000,000	$ 800,000,000
Unsecured notes payable under credit facilities									245,000,000	149,500,000
Capital leases									21,000,000	21,000,000
Net premium (discount) on debt									(6,100,000)
Deferred debt costs									(10,600,000)
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
Secured Debt [Member]
Debt Instrument [Line Items]
Deferred debt costs									$ (1,700,000)
Non-cash debt									$ 5,100,000
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
Three Point Two Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 250,000,000
Debt stated interest rate			3.25%
Debt instrument, issued at discount			99.16%
Debt instrument, interest rate, 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 instrument, face amount													$ 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%
Seven Point Four Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt instrument, face amount										$ 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%
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		$ 500,000,000					$ 500,000,000
Number of credit facility 6-month extensions | extensions	2
Line of credit facility, extension period	6 months
Line of credit facility, commitment fee percentage									0.15%	0.15%
Bids amount (up to)									$ 250,000,000	$ 250,000,000
Increase in credit facility amount (up to)									850,000,000	700,000,000
Unsecured notes payable under credit facilities									$ 245,000,000	$ 140,000,000
Unsecured Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points									0.90%	1.05%
Line of Credit [Member] | Short-Term Unsecured Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		$ 10,000,000												$ 20,000,000
Line of credit facility, commitment fee percentage		0.10%
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]
Unsecured notes payable under credit facilities									$ 0	$ 9,500,000
Interest Expense [Member] | Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on debt extinguishment				$ 2,000,000
Interest Expense [Member] | Three Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on debt extinguishment										$ (6,100,000)