Subsequent Events	12 Months Ended
Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 12, 2018, the Company entered into an agreement to purchase for $35.5 million, plus approximately $0.7 million of acquisition costs, an office building and the underlying ground located at 7316 Wisconsin Avenue in Bethesda, Montgomery County, Maryland and has an earnest money deposit of $3.5 million at risk. The purchase price will be funded through the Company's revolving credit facility. The Company anticipates closing the acquisition on or before January 12, 2019. On January 23, 2018, the Company sold, in an underwritten public offering, 3.0 million depositary shares, each representing 1/100^th of a share of 6.125% Series D Cumulative Redeemable Preferred Stock (the “Series D Stock”) and received net cash proceeds totaling approximately $72.6 million. The depositary shares may be redeemed at the Company’s option, in whole or in part, on or after January 23, 2023, at the $25.00 liquidation preference, plus accrued but unpaid dividends. The depositary shares pay an annual dividend of $1.53125 per share, equivalent to 6.125% of the $25.00 liquidation preference. The first dividend is scheduled to be paid on April 15, 2018, and cover the period from January 23, 2018, through March 31, 2018. The Series D Stock has no stated maturity, is not subject to any sinking fund or mandatory redemption and is not convertible into any other securities of the Company except in connection with certain changes of control or delisting events. Investors in the depositary shares generally have no voting rights, but will have limited voting rights if the Company fails to pay dividends for six or more quarters (whether or not declared or consecutive) and in certain other events. On January 26, 2018, the Company replaced its revolving credit facility, which was scheduled to expire on June 23, 2018, with a new credit facility. The new credit facility is comprised of a $75.0 million term facility that matures on January 26, 2023, and a $325.0 million revolving facility that matures on January 26, 2022, and can be extended for one year at the Company’s option, subject to satisfaction of certain conditions. The terms, conditions and covenants of the new credit facility are substantially the same as the existing credit facility. On February 22, 2018, the Company used the net proceeds from the sale of the Series D Stock, together with cash on hand, to redeem 3.0 million depositary shares, each representing 1/100^th of a share of Series C Stock at a price of $25.00 per depositary share, plus accrued dividends.

Subsequent Events

On January 12, 2018, the Company entered into an agreement to purchase for $35.5 million, plus approximately $0.7 million of acquisition costs, an office building and the underlying ground located at 7316 Wisconsin Avenue in Bethesda, Montgomery County, Maryland and has an earnest money deposit of $3.5 million at risk. The purchase price will be funded through the Company's revolving credit facility. The Company anticipates closing the acquisition on or before January 12, 2019.

On January 23, 2018, the Company sold, in an underwritten public offering, 3.0 million depositary shares, each representing 1/100^th of a share of 6.125% Series D Cumulative Redeemable Preferred Stock (the “Series D Stock”) and received net cash proceeds totaling approximately $72.6 million. The depositary shares may be redeemed at the Company’s option, in whole or in part, on or after January 23, 2023, at the $25.00 liquidation preference, plus accrued but unpaid dividends. The depositary shares pay an annual dividend of $1.53125 per share, equivalent to 6.125% of the $25.00 liquidation preference. The first dividend is scheduled to be paid on April 15, 2018, and cover the period from January 23, 2018, through March 31, 2018. The Series D Stock has no stated maturity, is not subject to any sinking fund or mandatory redemption and is not convertible into any other securities of the Company except in connection with certain changes of control or delisting events. Investors in the depositary shares generally have no voting rights, but will have limited voting rights if the Company fails to pay dividends for six or more quarters (whether or not declared or consecutive) and in certain other events.

On January 26, 2018, the Company replaced its revolving credit facility, which was scheduled to expire on June 23, 2018, with a new credit facility. The new credit facility is comprised of a $75.0 million term facility that matures on January 26, 2023, and a $325.0 million revolving facility that matures on January 26, 2022, and can be extended for one year at the Company’s option, subject to satisfaction of certain conditions. The terms, conditions and covenants of the new credit facility are substantially the same as the existing credit facility.

On February 22, 2018, the Company used the net proceeds from the sale of the Series D Stock, together with cash on hand, to redeem 3.0 million depositary shares, each representing 1/100^th of a share of Series C Stock at a price of $25.00 per depositary share, plus accrued dividends.