Business and Organization (Details)				3 Months Ended	9 Months Ended		12 Months Ended	47 Months Ended	48 Months Ended	59 Months Ended
Business and Organization (Details)	Nov. 09, 2016 USD ($)	Sep. 18, 2013 USD ($) shares	Dec. 18, 2012 USD ($) shares	Mar. 31, 2018	Sep. 30, 2017 USD ($) employee $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Nov. 09, 2016 USD ($) shares	Nov. 30, 2016 USD ($)	Nov. 08, 2017 USD ($)	Nov. 08, 2016 USD ($)	May 06, 2013 USD ($)
Class of Stock [Line Items]
Number of employees \| employee					0
Non-controlling interest investment in operating partnership					$ (248,000)	$ (213,000)
Number of common shares authorized for issuance (in shares) \| shares					400,000,000
Number of preferred shares authorized for issuance (in shares) \| shares					50,000,000		50,000,000
Preferred stock par value (in dollars per share) \| $ / shares					$ 0.01		$ 0.01
Proceeds from issuance of common stock							$ 220,568,000
Value of shares in offering												$ 1,650,000,000
Primary Offering
Class of Stock [Line Items]
Number of shares of common stock issued (in shares) \| shares								109,200,000
Proceeds from issuance of common stock								$ 1,100,000,000
Value of shares in offering												1,500,000,000
Value of shares remaining in prior offering											$ 150,000,000
Distribution Reinvestment Plan
Class of Stock [Line Items]
Value of shares in offering												$ 150,000,000
Value of shares allocated from primary offering to DRP	$ 150,000,000
Value of remaining shares available for issuance	$ 300,000,000							$ 300,000,000
Notice period served by board of directors to amend or terminate distribution reinvestment plan (in days)					10 days
Primary Offering Including Dividend Reinvestment Plan
Class of Stock [Line Items]
Proceeds from issuance of common stock									$ 1,100,000,000
Subsequent Event \| Distribution Reinvestment Plan
Class of Stock [Line Items]
Proceeds from issuance of common stock										$ 81,700,000
Subsequent Event \| Primary Offering Including Dividend Reinvestment Plan
Class of Stock [Line Items]
Proceeds from issuance of common stock										$ 1,200,000,000
Class A
Class of Stock [Line Items]
Number of common shares authorized for issuance (in shares) \| shares					320,000,000		320,000,000
Common stock par value (in dollars per share) \| $ / shares					$ 0.01		$ 0.01
Number of shares of common stock issued (in shares) \| shares			22,223
Proceeds from issuance of common stock			$ 200,000
Class A \| Northstar Realty
Class of Stock [Line Items]
Number of shares of common stock issued (in shares) \| shares		222,223
Proceeds from issuance of common stock		$ 2,000,000
Class T
Class of Stock [Line Items]
Number of common shares authorized for issuance (in shares) \| shares					80,000,000		80,000,000
Common stock par value (in dollars per share) \| $ / shares					$ 0.01		$ 0.01
Primary Beneficiary
Class of Stock [Line Items]
Limited partnership interest in the operating partnership (as a percent)					99.98%
Prior Advisor
Class of Stock [Line Items]
Non-controlling interest investment in operating partnership					$ 1,000		$ 1,000
Special Unit Holder
Class of Stock [Line Items]
Non-controlling interest investment in operating partnership					$ 1,000		$ 1,000
Colony NorthStar Credit Real Estate, Inc. \| NorthStar Real Estate Income II, Inc. \| Forecast
Class of Stock [Line Items]
LLC ownership interest				31.00%
Colony NorthStar Credit Real Estate, Inc. \| Colony Capital Operating Company, LLC \| Forecast
Class of Stock [Line Items]
LLC ownership interest				37.00%
Colony NorthStar Credit Real Estate, Inc. \| NorthStar Real Estate Income Trust, Inc. \| Forecast
Class of Stock [Line Items]
LLC ownership interest				32.00%

NorthStar Real Estate Income II, Inc. 10-Q