Share-based Compensation - Additional Information (Details)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2015 USD ($)	Aug. 31, 2014 shares	Aug. 31, 2011 USD ($)	Sep. 30, 2015 USD ($) unit / shares fungible_unit shares	Sep. 30, 2014 USD ($) shares	Sep. 30, 2015 USD ($) unit / shares fungible_unit $ / shares shares	Sep. 30, 2014 USD ($) shares	Dec. 31, 2014 USD ($) shares	Jun. 30, 2014 shares	Dec. 31, 2013 shares	Dec. 11, 2013	Dec. 31, 2012 shares	Dec. 17, 2012	Dec. 31, 2011 shares	Dec. 31, 2010 shares	Dec. 31, 2009 USD ($)	Jan. 01, 2008 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options				$ 1,500,000	$ 1,700,000	$ 4,900,000	$ 5,200,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of commencement of option vesting, from date of grant (in years)						1 year
Exercise price of options granted, low end of the range (in dollars per share) | $ / shares						$ 20.67
Exercise price of options granted, high end of the range (in dollars per share) | $ / shares						$ 137.18
Remaining weighted average contractual life of the options outstanding (in years)						3 years 6 months 24 days
Remaining weighted average contractual life of the options exercisable (in years)						3 years 4 months 11 days
Total unrecognized compensation cost related to unvested stock awards				8,800,000		$ 8,800,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)						3 years
Weighted average fair value of options granted during the period						$ 1,112,152		$ 2,841,678
Stock options | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)						5 years
Options vesting period (in years)						1 year
Stock options | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)						10 years
Options vesting period (in years)						5 years
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense						$ 5,663,196		9,658,019
Total unrecognized compensation cost related to unvested stock awards				$ 7,400,000		$ 7,400,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)						1 year 8 months 24 days
Annual award vesting rate, low end of range (as a percent)						15.00%
Annual award vesting rate, high end of range (as a percent)						35.00%
Fair value of restricted stock vested during the period						$ 7,400,000		5,500,000
Weighted average fair value of options granted during the period						$ 735,011		$ 1,141,675
Awards granted (in shares) | shares						6,053		9,550
Awards outstanding (in shares) | shares				3,003,315		3,003,315		3,000,979		2,994,197
LTIP units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of options granted during the period						$ 25,400,000		$ 33,200,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested stock awards				$ 4,600,000		$ 4,600,000
Weighted average period for recognition of compensation cost related to unvested stock awards (in years)						1 year
Share-based Compensation				$ 2,700,000	3,400,000	$ 19,000,000	13,700,000
Third Amendment and Restated 2005 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum fungible units that may be granted (in shares) | fungible_unit				17,130,000		17,130,000
Fungible units per share (in fungible units per share) | unit / shares				2.76		2.76
Shares that may be issued if equal to fungible units (shares) (less than) | shares				17,130,000		17,130,000
Fungible units (in shares) | shares				1,700,000		1,700,000
Third Amendment and Restated 2005 Stock Option and Incentive Plan | Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense				$ 1,900,000	2,000,000	$ 5,800,000	6,000,000
Stock options, stock appreciation rights and other awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fungible units per share (in fungible units per share) | unit / shares				0.77		0.77
Award expiration period (in years)						5 years
All other awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fungible units per share (in fungible units per share) | unit / shares				1.0		1.0
Award expiration period (in years)						10 years
2010 Notional Unit Long-Term Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					400,000		2,300,000
Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options	$ 31,700,000
Approximate amount of awards that may be earned by recipients after beginning of the second year if maximum performance achieved																$ 25,000,000
Approximate amount of awards that may be earned by recipients after beginning of the third year if maximum performance achieved																$ 25,000,000
Minimum stock price appreciation to earn maximum amount of awards (as a percent)																50.00%
LTIP units earned (in shares) | shares												327,416		385,583	366,815
Percentage of LTIP Units earned, vested (percent)											25.00%		50.00%
2010 Notional Unit Long-Term Compensation Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Approximate amount of LTIP units that may be earned by the recipients based on stock price appreciation																$ 15,000,000
2010 Notional Unit Long-Term Compensation Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Approximate amount of LTIP units that may be earned by the recipients based on stock price appreciation																$ 75,000,000
2011 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense				$ 600,000	$ 1,700,000	$ 4,500,000	$ 7,800,000
Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options						26,700,000
Approximate amount of LTIP units that may be earned by the recipients based on stock price appreciation			$ 85,000,000
LTIP units earned (in shares) | shares					280,454		280,454		560,908
Value of LTIP Units that could be earned expressed as percentage of outperformance amount in excess of the 30% benchmark (percent)			10.00%
Duration period return to stockholders exceeds 25% (in years)			3 years
Cumulative return to stockholders (percent)			25.00%
Return to stockholders			$ 85,000,000.0
Performance period (in years)			3 years
2014 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense				1,500,000		4,400,000
Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options						27,900,000
LTIP units earned (in shares) | shares		610,000
Performance period (in years)		3 years
Award period (in years)		3 years
Percentage of LTIP units that may be earned based on the Company's absolute total return to stockholders		66.67%
Percentage of LTIP units that may be earned based on relative total return to stockholders compared to constituents of the MSCI REIT index		33.33%
Deferred Stock Compensation Plan for Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense				$ 100,000	$ 100,000	$ 1,800,000	$ 1,500,000
Maximum percentage of the annual retainer fee, chairman fees and meeting fees that may be deferred by non-employee directors (percent)				100.00%		100.00%
Awards granted (in shares) | shares						8,644
Shares issued (in shares) | shares						5,640
Awards outstanding (in shares) | shares				84,348		84,348
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock available for issuance (shares) | shares																	500,000
Duration of each offering period starting the first day of each calendar quarter (in months)						3 months
Purchase price as a percentage of market value of the common stock (percent)				85.00%		85.00%
Shares of common stock issued (shares) | shares				85,317		85,317
Earned anytime after beginning of second year | 2011 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested			33.33%
Vested on August 31, 2014 | 2011 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested			50.00%
Vesting on August 31, 2015 | 2011 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested			50.00%
Earned any time after beginning of third year | 2011 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested			66.67%
Vesting on August 31, 2017 | 2014 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested		50.00%
Vesting on August 31, 2018 | 2014 Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of units vested		50.00%