Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
		Dec. 29, 2015	Sep. 30, 2017	Dec. 31, 2017		Nov. 17, 2017	Nov. 09, 2017	May 01, 2017	Dec. 31, 2016
Secured Debt [Member]
Debt Instrument [Line Items]
Underwriting offering amount				$ 320,107					$ 325,038
Unsecured Debt [Member]
Debt Instrument [Line Items]
Underwriting offering amount				$ 1,628,610					1,703,610
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt instrument increase in principal		$ 130,000
Debt Instrument Maturity Date				Apr. 05, 2023
Underwriting offering amount				$ 123,667					127,026
$300.0M 5.70% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date	[1]			May 01, 2017
Debt Instrument Interest Rate Stated Percentage								5.70%
Underwriting offering amount	[1]			$ 0					300,000
$450.0M 3.95% Guaranteed Notes due 2027 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date	[2]			Nov. 30, 2027
Debt Instrument Interest Rate Stated Percentage							3.95%
Underwriting offering amount				$ 450,000	[2]		$ 450,000		0	[2]
$250M 3.95% Guaranteed Notes due 2023 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date	[2]			Feb. 15, 2023
Debt Instrument Interest Rate Stated Percentage							3.95%
Underwriting offering amount	[2]			$ 350,000					250,000
Debt Instrument, Face Amount							$ 100,000
$325.0M 4.95% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date	[2]			Apr. 15, 2018
Debt Instrument Interest Rate Stated Percentage						4.95%
Underwriting offering amount	[2]			$ 0					325,000
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity Date	[3]			Mar. 30, 2035
Underwriting offering amount	[3]			$ 27,062					$ 27,062
Debt Instrument, Interest Rate Terms			LIBOR + 1.25%	LIBOR + 1.25%	[3]
Spread on variable rate			1.25%	1.25%	[3]

[1] 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.

[2] On November 17, 2017, the Company completed an offering of $450.0 million 3.95% Guaranteed Notes due 2027 (the “2027 Notes) and reopened the 3.95% Guaranteed Notes due 2023 (the “2023 Notes”) for an additional $100.0 million. On November 17, 2017, the Company redeemed the remaining outstanding balance of the 4.95% Guaranteed Notes due 2018 (the “2018 Notes”) ahead of its scheduled maturity. See below for further discussion.

[3] On September 30, 2017, the interest rate hedge contract for the Preferred Trust I expired. Subsequent to this expiration, the debt bears interest at a variable rate of LIBOR + 1.25%.