Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) - USD ($) $ in Millions			9 Months Ended
		Dec. 29, 2015	Sep. 30, 2017		May 01, 2017	Dec. 31, 2015
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt instrument increase in principal		$ 130.0
Debt instrument maturity date			Apr. 05, 2023
Effective interest rate			3.64%	[1]		3.64%
$300.0M 5.70% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument maturity date	[2]		May 01, 2017
Effective interest rate	[2]		5.68%
Debt Instrument Interest Rate Stated Percentage					5.70%

[1] This loan was assumed upon acquisition of the related properties on December 19, 2013. On December 29, 2015, the Company refinanced the debt increasing the principal balance to $130.0 million and extended the scheduled maturity date from January 6, 2016 to April 5, 2023.  The effective interest rate as of December 31, 2015 was 3.64%.  A default under this loan will also constitute a default under the loan secured by Two Commerce Square. This loan is also secured by a lien on Two Commerce Square.

[2] On May 1, 2017, the entire principal balance of the unsecured 5.70% Guaranteed Notes was repaid upon maturity. Available cash balances and the Credit Facility (as defined below) were used to fund the repayment of the unsecured notes.