Unsecured Line of Credit (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) yr mo	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,000,000
Line of Credit Facility, Expiration Date	Jul. 26, 2018
Line of Credit Facility, Frequency of Commitment Fee Payment	quarterly
Line of Credit Facility, Amount Outstanding	$ 0	$ 0
Leverage Ratio, Maximum	60.00%
Leverage Ratio Maximum Increasing Limit	65.00%
Leverage Ratio Reduction Limit	60.00%
Leverage Ratio Reduction Period, Years | yr	1
Maximum Secured Debt Leverage Ratio	55.00%
Fixed Charge Coverage Ratio Minimum.	1.40
Maximum Unsecured Debt Leverage Ratio	60.00%
Unsecured Debt Leverage Ratio Maximum Increasing Limit	65.00%
Unsecured Debt Leverage Ratio Reduction Limit	60.00%
Unsecured Debt Leverage Ratio Reduction Period, Years | yr	1
Unsecured Debt Interest Coverage Ratio Minimum.	1.75
Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Amount added to Federal Funds Rate to Calculate Interest Rate	0.50%
LIBOR Period Used to Calculate Interest Rate (in months) | mo	1
Amount added to LIBOR to Calculate Interest Rate	1.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,500,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Maximum [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.70%
Maximum [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.70%
Minimum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.125%
Minimum [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.925%
Minimum [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Rate Based on Current Credit Rating [Member]
Line of Credit Facility [Line Items]
Line Of Credit Facility Extension Fee	0.15%
Rate Based on Current Credit Rating [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.00%
Rate Based on Current Credit Rating [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%