Debt - Unsecured Revolving Credit Facility and Term Loan and Interst Expense (Details) $ in Thousands	Feb. 08, 2019 USD ($) extension_option	Feb. 01, 2016 USD ($) extension_option	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Outstanding amounts			$ 565,000	$ 495,000
Outstanding borrowings			559,382	489,765
Revolving Credit Facility | New Revolving Facility
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 600,000
Number of extension options | extension_option	2
Line of credit facility, extension period	6 months
Revolving Credit Facility | New Revolving Facility | Minimum
Line of Credit Facility [Line Items]
Facility fee on revolving commitment fees (percent)	0.15%
Facility fee on revolving commitment fee based on investment grade ratings (percent)	0.125%
Revolving Credit Facility | New Revolving Facility | Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.10%
Revolving Credit Facility | New Revolving Facility | Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.10%
Revolving Credit Facility | New Revolving Facility | Maximum
Line of Credit Facility [Line Items]
Facility fee on revolving commitment fees (percent)	0.35%
Facility fee on revolving commitment fee based on investment grade ratings (percent)	0.30%
Revolving Credit Facility | New Revolving Facility | Maximum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.55%
Revolving Credit Facility | New Revolving Facility | Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.55%
Letter of Credit
Line of Credit Facility [Line Items]
Subfacility capacity as percentage of available revolving commitments (percent)	10.00%
Swingline Loan
Line of Credit Facility [Line Items]
Subfacility capacity as percentage of available revolving commitments (percent)	10.00%
Prior Credit Facility
Line of Credit Facility [Line Items]
Outstanding amounts			65,000	95,000
Outstanding borrowings			65,000	95,000
Prior Credit Facility | Prior Revolving Facility
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity		$ 400,000
Line of credit facility, uncommitted incremental facility limit		$ 250,000
Number of extension options | extension_option		2
Line of credit facility, extension period		6 months
Prior Credit Facility | Prior Term Loan
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity		$ 100,000
Line of credit facility, prepayment premium in first year		2.00%
Line of credit facility, prepayment premium in second year		1.00%
Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount			300,000	300,000
Unsecured Debt | New Term Loan Feb 2019
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 200,000
Unsecured Debt | New Term Loan Feb 2019 | Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	0.50%
Unsecured Debt | New Term Loan Feb 2019 | Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.50%
Unsecured Debt | New Term Loan Feb 2019 | Maximum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	1.20%
Unsecured Debt | New Term Loan Feb 2019 | Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate (percent)	2.20%
Prior Term Loan
Line of Credit Facility [Line Items]
Outstanding amounts			200,000	100,000
Outstanding borrowings			$ 198,661	$ 99,612