Unsecured Credit Facility (Details) $ in Thousands		12 Months Ended
	Apr. 24, 2017 USD ($) mo	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) yr	Apr. 23, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding		$ 45,000	$ 45,000		$ 0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,500,000			$ 1,000,000
Competitive bid quote	65.00%
Maximum Leverage Ratio		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	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		1.75	1
Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 500,000			$ 500,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.15%
Line of Credit Facility, Frequency of Commitment Fee Payment	4
minimum borrowing amount	$ 50,000
Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.30%
Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.10%
Current credit rating [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.15%
Interest Rate Based on LIBOR or CDOR [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.55%
Interest Rate Based on LIBOR or CDOR [Member] | Maximum [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.75%
Interest Rate Based on LIBOR or CDOR [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.775%
Interest Rate Based on LIBOR or CDOR [Member] | Minimum [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.85%
Interest Rate Based on LIBOR or CDOR [Member] | Current credit rating [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate		0.875%	0.875%
Interest Rate Based on LIBOR or CDOR [Member] | Current credit rating [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate		0.95%	0.95%
Alternative Base Interest Rate Calculation [Member] | Revolving Credit Facility [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%
Alternative Base Interest Rate Calculation [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.55%
Alternative Base Interest Rate Calculation [Member] | Maximum [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.75%
Alternative Base Interest Rate Calculation [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Alternative Base Interest Rate Calculation [Member] | Minimum [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Alternative Base Interest Rate Calculation [Member] | Current credit rating [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate		0.00%	0.00%
Alternative Base Interest Rate Calculation [Member] | Current credit rating [Member] | Delayed Draw Facility [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate		0.00%	0.00%