Basis of Presentation and Significant Accounting Policies (Income Taxes) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 30, 2018
Tax Treatment of Dividend
Minimum percentage of taxable income distributed as dividends	90.00%
Percentage of taxable income distributed	100.00%
Deferred tax assets related to net operating loss carryforward	$ 109,949,000	$ 69,209,000
Deferred tax liabilities	$ 28,676,000	13,697,000
Increase in income tax expense		$ 34,800,000
Effective income tax reconcilation, percent	8.20%	13.30%	1.40%
Income tax expense relating to purchase price fair value adjustment	$ 16,771,000
Income tax expense	37,633,000	$ 42,375,000	$ 7,923,000
Net basis difference of assets and liabilities between tax basis and GAAP basis	$ 1,900,000,000
Ordinary Income
Tax Treatment of Dividend
Percentage of taxable income distributed	91.70%		83.50%
Long Term Capital Gain
Tax Treatment of Dividend
Percentage of taxable income distributed	8.30%		16.50%
220 Central Park South
Tax Treatment of Dividend
Deferred tax assets related to net operating loss carryforward	$ 42,035,000
Income tax expense	$ 13,888,000
Farley Office and Retail Building
Tax Treatment of Dividend
Equity method additional ownership percentage	44.90%			44.90%
Income tax expense	$ 16,771,000