Organization (Details) ft² in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 ft² Real_Estate_Properties series yr shares	Dec. 31, 2017 shares
Real Estate Properties [Line Items]
General and limited partnership interest in the operating partnership (percent)	89.70%	89.70%
Restriction on redemption of OP units from date of issuance (years) | yr	1
One OP unit is equivalent to one share of Common Stock (in shares) | shares	1
OP unit conversion rate (in shares) | shares	1
Number Of Series Of Preferred Units Outstanding | series	1
Commercial Real Estate Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	200
Net Rentable Area (in sf) | ft²	52.7
Total Properties Under Construction [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	14
Net Rentable Area (in sf) | ft²	7.5
Total Office Properties [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	180
Office Properties Under Construction [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	12
Residential Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	6
Residential Properties Under Construction [Member]
Real Estate Properties [Line Items]
Number of real estate properties	2
Retail Properties [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	13
Hotel Property [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	1
Series B Cumulative Redeemable Preferred Stock [Member]
Real Estate Properties [Line Items]
Ratio of depository shares to shares of Series B Preferred Stock	0.01
Series B, Shares Outstanding (in shares) | shares	80,000	80,000
Series B, Dividend Rate, Percentage	5.25%
Depository shares of Series B Cumulative Redeemable Preferred [Member]
Real Estate Properties [Line Items]
Series B, Shares Outstanding (in shares) | shares	8,000,000
Series B Preferred Units [Member]
Real Estate Properties [Line Items]
Series B, Shares Outstanding (in shares) | shares	80,000