Fair Value Measurements (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value of Financial Instruments
Marketable securities	$ 28,802,000	$ 85,110,000
Interest rate cap and swap agreements (included in other assets)	13,727,000	21,090,000
Debt and preferred equity investments	2,020,739,000	1,640,412,000
Equity method investments	2,045,796,000	1,890,186,000
Additional amount of financing receivables included in other assets	65,600,000	99,500,000
Accrued Interest Payable and Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in accrued interest payable and other liabilities)		1,000
Carrying Value
Fair Value of Financial Instruments
Debt and preferred equity investments	2,020,739,000	1,640,412,000
Fixed rate debt	5,154,810,000	5,452,084,000
Variable rate debt	1,321,813,000	1,105,585,000
Total	6,476,623,000	6,557,669,000
Fair Value
Fair Value of Financial Instruments
Marketable securities	28,802,000	85,110,000
Total	6,713,093,000	6,832,604,000
Estimated fair value of debt and preferred equity investments, low end of range	2,000,000,000	1,600,000,000
Estimated fair value of debt and preferred equity investments, high end of range	2,200,000,000	1,800,000,000
Level 1
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	0	0
Level 1 | Accrued Interest Payable and Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in accrued interest payable and other liabilities)		0
Level 2
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	13,727,000	21,090,000
Level 2 | Accrued Interest Payable and Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in accrued interest payable and other liabilities)		1,000
Level 3
Fair Value of Financial Instruments
Marketable securities	0	0
Interest rate cap and swap agreements (included in other assets)	0	0
Level 3 | Accrued Interest Payable and Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in accrued interest payable and other liabilities)		0
Level 3 | Fair Value
Fair Value of Financial Instruments
Fixed rate debt	5,398,870,000	5,722,494,000
Variable rate debt	1,314,223,000	1,110,110,000
Equity marketable securities
Fair Value of Financial Instruments
Marketable securities	0	48,315,000
Equity marketable securities | Level 1
Fair Value of Financial Instruments
Marketable securities	0	48,315,000
Commercial mortgage-backed securities
Fair Value of Financial Instruments
Marketable securities	28,802,000	36,795,000
Commercial mortgage-backed securities | Level 2
Fair Value of Financial Instruments
Marketable securities	28,802,000	36,795,000
Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Fair Value of Financial Instruments
Debt and preferred equity investments	250,200,000	$ 0	$ 0
Total fixed rate | Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Fair Value of Financial Instruments
Notes receivable	$ 250,000,000