Earnings Per Share - Earnings Allocable to Common Shares (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) trading_day $ / shares shares
Numerator for basic earnings per share:
Net income (loss) attributable to iStar Inc. and allocable to common shareholders | $	$ (2,069)	$ (7,343)	$ (46,850)	$ 337,807
Numerator for diluted earnings per share:
Net income (loss) attributable to iStar Inc. and allocable to common shareholders | $	$ (2,069)	$ (7,343)	$ (46,850)	$ 344,557
Denominator for basic and diluted earnings per share:
Weighted average common shares outstanding for basic earnings per common share (in shares)	75,033	62,168	76,232	64,624
Add: Effect of assumed shares issued under treasury stock method for restricted stock units (in shares)	0	0	0	114
Add: Effect of series J convertible perpetual preferred stock (in shares)	0	0	0	16,138
Weighted average common shares outstanding for diluted earnings per common share (in shares)	75,033	62,168	76,232	80,876
Basic earnings per common share:
Net income (loss) allocable to common shareholders (in dollars per share) | $ / shares	$ (0.03)	$ (0.12)	$ (0.61)	$ 5.23
Diluted earnings per common share:
Net income (loss) allocable to common shareholders (in dollars per share) | $ / shares	$ (0.03)	$ (0.12)	$ (0.61)	$ 4.26
Series J convertible perpetual preferred stock		16,306
3.125% Senior Convertible Notes
Diluted earnings per common share:
Stated interest rates (as a percent)	3.125%	3.125%	3.125%	3.125%
Threshold consecutive trading days | trading_day				40