Subsequent Events (Details)								1 Months Ended	2 Months Ended	3 Months Ended		12 Months Ended
	Mar. 05, 2018 shares	Feb. 28, 2018 USD ($)	Feb. 06, 2018 USD ($) $ / ft²	Jan. 05, 2018 shares	Dec. 27, 2017 USD ($) shares	Aug. 07, 2017 USD ($) shares	Jun. 01, 2017 USD ($) shares	Feb. 06, 2018 $ / ft² $ / shares	Mar. 05, 2018 $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 USD ($) $ / shares	Nov. 30, 2015	Dec. 31, 2014 $ / ft²
Subsequent Event [Line Items]
Dividends declared (in dollars per share) | $ / shares												$ 0.08	$ 0.08	$ 0.28
Base rent per square foot (in dollars per square foot) | $ / ft²																18
AC 4% PIK Note [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate											4.00%	4.00%			4.00%
Loan from AC [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate					1.60%
AC 1.6% Note [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate											1.60%	1.60%
Class A [Member]
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares)											9,949,482	9,949,482	10,369,601
Restricted Stock Awards [Member]
Subsequent Event [Line Items]
Number of shares with accelerated vesting (in shares)					144,650	201,120					144,650	345,770
Compensation record expenses due to accelerated vesting | $					$ 1,300,000	$ 1,800,000					$ 1,300,000	$ 6,800,000		$ 3,500,000
Restricted Stock Awards [Member] | AC [Member]
Subsequent Event [Line Items]
Number of shares with accelerated vesting (in shares)							420,240
Compensation record expenses due to accelerated vesting | $							$ 3,700,000
Restricted Stock Awards [Member] | Scenario, Forecast [Member]
Subsequent Event [Line Items]
Compensation record expenses due to accelerated vesting | $										$ 187,000
Subsequent Event [Member] | AC [Member]
Subsequent Event [Line Items]
Base rent per square foot (in dollars per square foot) | $ / ft²			36.71					36.71
Electricity bill per square foot (in dollars per square foot) | $ / ft²			3					3
Common Stock, Voting Powers Owned, Percentage	2.00%								2.00%
Common Stock, Shares Outstanding, Percentage	13.00%								13.00%
Subsequent Event [Member] | G.research LLC [Member]
Subsequent Event [Line Items]
Renewal of research service agreement | $			$ 1,500,000
Subsequent Event [Member] | G.research LLC [Member] | Fund Advisor [Member]
Subsequent Event [Line Items]
Renewal of research service agreement | $			$ 1,500,000
Subsequent Event [Member] | Repurchase of Common Stock [Member]
Subsequent Event [Line Items]
Stock repurchased (in shares)									59,611
Average price per share of repurchased shares (in dollars per share) | $ / shares									$ 29.33
Shares available to be repurchsed under the plan (in shares)	614,683								614,683
Subsequent Event [Member] | Quarterly Dividend [Member]
Subsequent Event [Line Items]
Dividends declared date								Feb. 06, 2018
Dividends declared (in dollars per share) | $ / shares								$ 0.02
Dividends payable date								Mar. 27, 2018
Dividends record date								Mar. 13, 2018
Subsequent Event [Member] | AC 1.6% Note [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate		1.60%
Repayment of debt | $		$ 15,000,000
Subsequent Event [Member] | Class A [Member]
Subsequent Event [Line Items]
Number of Shares to Be Received in Exchange for Each Share Tendered	1.35
Number of shares to be delivered in exchange offer	660,000
Subsequent Event [Member] | Class A [Member] | AC [Member]
Subsequent Event [Line Items]
Number of shares validly tendered and accepted	490,000
Common stock, shares outstanding (in shares)	3,700,000								3,700,000
Subsequent Event [Member] | Restricted Stock Awards [Member]
Subsequent Event [Line Items]
Number of shares with accelerated vesting (in shares)				19,400