Shareholders' Equity / Partners' Capital - At the Market Common Share Offering Program (Details) - USD ($) $ / shares in Units, shares in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Aug. 31, 2017	Nov. 30, 2016
Class of Stock [Line Items]
Proceeds from issuance of shares/units	$ 0	$ 694,765,000
Class A common shares/units
Class of Stock [Line Items]
Stock issuance costs	0	$ 10,444,000
At the Market - Common Share Offering Program | Class A common shares/units
Class of Stock [Line Items]
Shares available for future issuance, value	$ 500,000,000			$ 400,000,000
Shares issued during period (in shares)		2.0
Proceeds from issuance of shares/units		$ 46,200,000
Shares issued, price per share (in dollars per share)		$ 22.74
Proceeds from issuance of common stock, net of issuance costs		$ 45,600,000
Stock issuance costs		$ 600,000
Shares authorized for future issuance, value			$ 500,000,000