Significant Accounting Policies - Investments in Unconsolidated Joint Ventures/Deferred Lease Costs/Revenue Recognition/Income Taxes (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) joint_venture $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares
Schedule of Equity Method Investments [Line Items]
Number of joint ventures with performance guarantees under master leases | joint_venture	2
Deferred Lease Costs [Abstract]
Portion of compensation capitalized	$ 16,400,000	$ 15,400,000	$ 15,400,000
Estimated average lease term (in years)	7 years
Days past due for income recognition on debt and preferred equity investments to be suspended	90 days
Loan reserves recorded	$ 0	0	0
Income taxes
Federal, state and local tax provision	4,300,000	2,800,000	3,100,000
Interest Expense Increase (Decrease) Assumed Above Market Rate Mortgage Amortization	$ (800,000)	$ (2,800,000)	$ (2,300,000)
Dividends per share paid (in dollars per share) | $ / shares	$ 3.100000	$ 2.880000	$ 2.400000
Ordinary Income Dividend
Income taxes
Dividends per share paid (in dollars per share) | $ / shares	1.236708	2.482236	0.00
Capital Gains Dividend
Income taxes
Dividends per share paid (in dollars per share) | $ / shares	$ 1.863292	$ 0.40	$ 2.40000