Unsecured Credit Facility (Details) $ in Thousands					4 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Apr. 23, 2018 USD ($) extension_options	Dec. 31, 2017 USD ($)	Jan. 03, 2017 USD ($)	Apr. 22, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Unsecured term loans	$ 447,583		$ 547,270			$ 447,583		$ 547,270
Revolving line of credit	$ 126,000		$ 216,000			126,000		216,000
Repayment of unsecured term loan						100,000	$ 0
Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	1.20%		1.30%
Unsecured term loans
Line of Credit Facility [Line Items]
Unsecured term loans	$ 450,000		$ 550,000			450,000		550,000
Capitalized loan fees, net of accumulated amortization	(2,417)		(2,730)			(2,417)		(2,730)
Term loans, net	$ 447,583		$ 547,270			447,583		547,270
Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	1.70%		1.70%
Principal amount				$ 200,000
Fixed rate debt | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Unsecured term loans	$ 250,000		$ 250,000			$ 250,000		$ 250,000
Interest rate (as a percent)	3.20%		3.30%			3.20%		3.30%
Fixed rate debt | Term Loan Due 2023
Line of Credit Facility [Line Items]
Unsecured term loans	$ 200,000		$ 200,000			$ 200,000		$ 200,000
Interest rate on term loan (as a percent)	2.96%		2.96%
Variable rate debt | Unsecured credit facility term loan due 2018
Line of Credit Facility [Line Items]
Unsecured term loans	$ 0		$ 100,000			$ 0		$ 100,000
Interest rate (as a percent)	0.00%		2.93%			0.00%		2.93%
Variable rate debt | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Revolving line of credit	$ 126,000		$ 216,000			$ 126,000		$ 216,000
Interest rate (as a percent)	3.14%		2.92%			3.14%		2.92%
Two $100,00 and one $50,000 interest rate swaps maturing in 2021
Line of Credit Facility [Line Items]
Variable rate debt swapped to fixed rate	$ 250,000					$ 250,000
Fixed interest rate (as a percent)	2.00%					2.00%
Two $100,00 and one $50,000 interest rate swaps maturing in 2021 | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable rate debt swapped to fixed rate	$ 250,000					$ 250,000
Fixed interest rate (as a percent)	2.00%					2.00%
Two $100,000 interest rate swaps maturing in 2018
Line of Credit Facility [Line Items]
Variable rate debt swapped to fixed rate	$ 200,000					$ 200,000
Fixed interest rate (as a percent)	1.26%					1.26%
Two $100,000 interest rate swaps maturing in 2018 | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable rate debt swapped to fixed rate	$ 200,000					$ 200,000
Fixed interest rate (as a percent)	1.26%					1.26%
LIBOR | Term Loan Due 2023
Line of Credit Facility [Line Items]
Reference rate for variable interest rate				LIBOR
LIBOR | Two $100,00 and one $50,000 interest rate swaps maturing in 2021
Line of Credit Facility [Line Items]
Reference rate for variable interest rate	one-month floating rate LIBOR
LIBOR | Two $100,00 and one $50,000 interest rate swaps maturing in 2021 | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Reference rate for variable interest rate	LIBOR
LIBOR | Two $100,000 interest rate swaps maturing in 2018
Line of Credit Facility [Line Items]
Reference rate for variable interest rate	one-month floating rate LIBOR
LIBOR | Two $100,000 interest rate swaps maturing in 2018 | Term Loan Due 2023
Line of Credit Facility [Line Items]
Reference rate for variable interest rate	LIBOR
Minimum | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	1.20%		1.30%
Minimum | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	1.70%
Minimum | LIBOR | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)				1.70%
Maximum | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	1.70%		2.20%
Maximum | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)	2.55%
Maximum | LIBOR | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)				2.55%
2018 Wells Fargo and KeyBank syndicate | Unsecured Credit Facility
Line of Credit Facility [Line Items]
Aggregate borrowing capacity		$ 1,100,000
Additional borrowing capacity		500,000
Maximum borrowing capacity		1,600,000
2018 Wells Fargo and KeyBank syndicate | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Principal amount		250,000
2018 Wells Fargo and KeyBank syndicate | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Aggregate borrowing capacity		$ 850,000
Number of extension options | extension_options		2
Revolving line of credit, period of extension of maturity (in years)		6 months
Revolving line of credit, extension fee as a percentage of commitment amount		0.075%
2018 Wells Fargo and KeyBank syndicate | Minimum | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Facility fee (as a percent)		0.15%
2018 Wells Fargo and KeyBank syndicate | Minimum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.20%
2018 Wells Fargo and KeyBank syndicate | Minimum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.05%
2018 Wells Fargo and KeyBank syndicate | Maximum | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Facility fee (as a percent)		0.30%
2018 Wells Fargo and KeyBank syndicate | Maximum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.70%
2018 Wells Fargo and KeyBank syndicate | Maximum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.50%
KeyBank and Wells Fargo syndicate related to previous credit facility | Unsecured Credit Facility
Line of Credit Facility [Line Items]
Aggregate borrowing capacity					$ 1,200,000
KeyBank and Wells Fargo syndicate related to previous credit facility | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Principal amount					250,000
KeyBank and Wells Fargo syndicate related to previous credit facility | Unsecured credit facility term loan due 2018
Line of Credit Facility [Line Items]
Principal amount					200,000
Repayment of unsecured term loan		$ 100,000						$ 100,000
KeyBank and Wells Fargo syndicate related to previous credit facility | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Aggregate borrowing capacity					$ 750,000
KeyBank and Wells Fargo syndicate related to previous credit facility | Minimum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					1.30%
KeyBank and Wells Fargo syndicate related to previous credit facility | Minimum | LIBOR | Unsecured credit facility term loan due 2018
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					1.45%
KeyBank and Wells Fargo syndicate related to previous credit facility | Minimum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					1.35%
KeyBank and Wells Fargo syndicate related to previous credit facility | Maximum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					2.20%
KeyBank and Wells Fargo syndicate related to previous credit facility | Maximum | LIBOR | Unsecured credit facility term loan due 2018
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					2.20%
KeyBank and Wells Fargo syndicate related to previous credit facility | Maximum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)					2.25%
Investment grade rated | Minimum | LIBOR | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)				1.50%
Investment grade rated | Maximum | LIBOR | Term Loan Due 2023
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)				2.45%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Minimum | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Facility fee (as a percent)		0.125%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Minimum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		0.90%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Minimum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		0.825%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Maximum | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Facility fee (as a percent)		0.30%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Maximum | LIBOR | Unsecured credit facility term loan due 2021
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.75%
Investment grade rated | 2018 Wells Fargo and KeyBank syndicate | Maximum | LIBOR | Unsecured credit facility revolving line of credit
Line of Credit Facility [Line Items]
Variable interest rate spread (as a percent)		1.55%