Debt (Narrative) (Details)			1 Months Ended			3 Months Ended	12 Months Ended
	Mar. 30, 2016 USD ($) extensions	Mar. 27, 2016 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 01, 2016	May 31, 2016 USD ($)	Feb. 28, 2015 USD ($)
Debt Instrument [Line Items]
Debt, net						$ 2,015,557,000
Debt service guaranty liability			$ 67,125,000			67,125,000	$ 67,125,000
Medium term notes, matured			$ 75,000,000
Debt interest rate during period			5.50%
Debt instruments collateral value			$ 700,000,000			700,000,000	700,000,000
Unsecured notes payable under credit facilities			245,000,000			230,900,000	245,000,000
Obligations under capital leases			21,000,000			21,000,000	$ 21,000,000
Net premium/(discount) on debt						(5,900,000)
Deferred finance costs, net						(10,100,000)
Non-cash debt						$ 4,900,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility	$ 500,000,000			$ 500,000,000						$ 500,000,000
Number of credit facility 6-month extensions | extensions	2
Line of credit facility, extension period	6 months
Facility fees, basis points						0.15%	0.15%
Bids amount (up to)						$ 250,000,000
Maximum increase in credit facility amount (up to)						850,000,000
Unsecured notes payable under credit facilities			245,000,000			226,000,000	$ 245,000,000
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Unsecured notes payable under credit facilities			$ 0			$ 4,900,000	$ 0
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt coverage ratio						1.4
Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		$ 10,000,000
Facility fees, basis points						0.10%	0.10%
Fixed interest rate loan period (in days)		30 days
Unused facility fees, basis points						0.50%	0.10%
Three Point Two Five Senior Unsecured Notes [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Debt payable				$ 250,000,000
Debt stated interest rate				3.25%
Debt issued discount rate				99.16%
Debt 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 payable									$ 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%
London Interbank Offered Rate (LIBOR) [Member] | Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points						0.90%	0.90%
London Interbank Offered Rate (LIBOR) [Member] | Short-Term Unsecured Facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Borrowing margin over LIBOR, basis points						1.25%	1.25%
Interest Expense [Member] | Four Point Five Secured Notes [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt					$ 2,000,000