Significant Accounting Policies (Investments in Unconsolidated Joint Ventures/Deferred Lease Costs/Revenue Recognition/Income Taxes)(Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) joint_venture	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Deferred Lease Costs [Abstract]
Portion of compensation capitalized	$ 15.4	$ 15.4	$ 15.1
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
Income taxes
Federal, state and local tax provision	$ 2.8	$ 3.1	$ 7.8
Joint venture
Schedule of Equity Method Investments [Line Items]
Number of joint ventures with performance guarantees under master leases | joint_venture	2