Fair Value Measurements (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair Value of Financial Instruments
Marketable securities	$ 32,049,000	$ 21,429,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	1,329,000	1,959,000
Debt and preferred equity investments	1,304,839,000	1,348,434,000
Carrying Value
Fair Value of Financial Instruments
Fixed rate debt	5,599,960,000	4,922,725,000
Variable rate debt	1,319,948,000	1,597,695,000
Notional Value	6,919,908,000	6,520,420,000
Fair Value
Fair Value of Financial Instruments
Notional Value	7,214,402,000	6,891,738,000
Estimated fair value of debt and preferred equity investments, low end of range	1,300,000,000
Estimated fair value of debt and preferred equity investments, high end of range	1,400,000,000
Level 3 | Carrying Value
Fair Value of Financial Instruments
Debt and preferred equity investments	1,304,839,000	1,348,434,000
Level 3 | Fair Value
Fair Value of Financial Instruments
Fixed rate debt	5,886,980,000	5,334,244,000
Variable rate debt	1,327,422,000	1,557,494,000
Equity marketable securities | Level 1
Fair Value of Financial Instruments
Marketable securities	4,307,000	2,202,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	0	0
Commercial mortgage-backed securities | Level 2
Fair Value of Financial Instruments
Marketable securities	24,419,000	15,575,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	1,329,000	1,959,000
Rake bonds | Level 3
Fair Value of Financial Instruments
Marketable securities	3,323,000	3,652,000
Interest rate swap agreements (included in accrued interest payable and other liabilities)	$ 0	$ 0