Significant Accounting Policies - Investments in Unconsolidated Joint Ventures/Deferred Lease Costs/Revenue Recognition/Income Taxes (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) joint_venture $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares
Schedule of Equity Method Investments [Line Items]
Number of joint ventures with performance guarantees under master leases | joint_venture	1
Deferred Lease Costs [Abstract]
Portion of compensation capitalized	$ 15,700,000	$ 16,400,000	$ 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 loss and other investment reserves, net of recoveries	$ 6,839,000	0	0
Income taxes
Federal, state and local tax provision	2,800,000	4,300,000	2,800,000
Interest Expense Increase (Decrease) Assumed Above Market Rate Mortgage Amortization	$ 0	$ (800,000)	$ (2,800,000)
Dividends per share paid (in dollars per share) | $ / shares	$ 3.25	$ 3.10	$ 2.88
Ordinary Income Dividend
Income taxes
Dividends per share paid (in dollars per share) | $ / shares	1.46	1.24	2.48
Capital Gains Dividend
Income taxes
Dividends per share paid (in dollars per share) | $ / shares	$ 1.79	$ 1.86	$ 0.40