Fair Value Measurements (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Marketable securities	$ 39,293,000	$ 32,049,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	14,114,000	1,329,000
Debt and preferred equity investments	1,432,951,000	1,304,839,000
Carrying Value
Fair Value of Financial Instruments
Fixed rate debt	6,133,200,000	5,599,960,000
Variable rate debt	2,603,152,000	1,319,948,000
Long term debt	8,736,352,000	6,919,908,000
Fair Value
Fair Value of Financial Instruments
Long term debt	9,164,524,000	7,214,402,000
Estimated fair value of debt and preferred equity investments, low end of range	1,400,000,000	1,300,000,000
Estimated fair value of debt and preferred equity investments, high end of range	1,600,000,000	1,400,000,000
Level 1
Fair Value of Financial Instruments
Marketable securities	4,315,000	4,307,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	0	0
Level 2
Fair Value of Financial Instruments
Marketable securities	34,978,000	24,419,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	14,114,000	1,329,000
Level 3
Fair Value of Financial Instruments
Marketable securities	0	3,323,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	0	0
Level 3 | Carrying Value
Fair Value of Financial Instruments
Debt and preferred equity investments	1,432,951,000	1,304,839,000
Level 3 | Fair Value
Fair Value of Financial Instruments
Fixed rate debt	6,536,977,000	5,886,980,000
Variable rate debt	$ 2,627,547,000	$ 1,327,422,000