Significant Accounting Policies - Investment in Commercial Real Estate Properties (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Apr. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Rental revenue decrease from amortization of acquired leases		$ 0.9		$ 6.2	$ 0.9	$ 14.5
719 Seventh Avenue
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Write down of value in leasehold interest			$ 46.3
719 Seventh Avenue | Mathias | Related party
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from sale of property	$ 30.5
760 Madison Avenue
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Write down of value in leasehold interest		13.7
Interest capitalized		$ 13.2
Minimum | Below-Market Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		1 year			1 year
Minimum | Above-Market Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		1 year			1 year
Minimum | In-Place Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		1 year			1 year
Minimum | Building
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)		3 years			3 years
Maximum | Below-Market Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		15 years			15 years
Maximum | Above-Market Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		15 years			15 years
Maximum | In-Place Leases
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of other intangible assets (in years)		15 years			15 years
Maximum | Building
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)		40 years			40 years