Corporate Indebtedness (Details) - USD ($)	1 Months Ended				12 Months Ended
	Jul. 31, 2015	Jan. 31, 2015	Nov. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2015
Corporate Indebtedness
Increase in credit facility	$ 500,000,000
Revolving credit facility					$ 994,000,000	$ 385,000,000
Debt disclosures by scheduled maturity date
Accreted Balance					1,988,106,000	1,788,001,000
Scheduled amortization and principal repayments
2016					512,298,000
2017					7,411,000
2018					257,799,000
2019					941,207,000
2020					1,252,637,000
Thereafter					764,380,000
Total amortization of debt and principal repayments					3,735,732,000
Interest expense
Interest expense					119,360,000	129,427,000	$ 133,853,000
Interest income					(18,000)	(71,000)	(8,000)
Interest expense, net					119,342,000	129,356,000	133,845,000
Interest capitalized					1,107,000	3,753,000	$ 0
Revolving credit facility
Corporate Indebtedness
Increase in credit facility	400,000,000
Maximum borrowing capacity	1,600,000,000.0				1,600,000,000.0
Maximum borrowing capacity, optional expansion					3,000,000,000.0
Revolving credit facility					$ 994,000,000
Debt disclosures by scheduled maturity date
Effective Rate (as a percent)					1.45%
Scheduled amortization and principal repayments
2016					$ 0
2017					0
2018					0
2019					0
2020					994,000,000
Thereafter					0
Total amortization of debt and principal repayments					$ 994,000,000
Revolving credit facility | Minimum
Corporate Indebtedness
Facility fee on total commitments (as a percent)					0.125%
Facility fee on total commitments, payable quarterly in arrears (as a percent)					0.25%
Revolving credit facility | Maximum
Corporate Indebtedness
Facility fee on total commitments (as a percent)					0.30%
Term loan
Corporate Indebtedness
Increase in credit facility	100,000,000		$ 50,000,000	$ 383,000,000
Maximum borrowing capacity	$ 933,000,000.0		$ 833,000,000		$ 933,000,000			$ 783,000,000
Decrease in interest-rate margin (as a percent)		0.20%		0.25%
Decrease in facility fee (as a percent)		0.05%
Debt disclosures by scheduled maturity date
Effective Rate (as a percent)					1.67%
Scheduled amortization and principal repayments
2016					$ 0
2017					0
2018					0
2019					933,000,000
2020					0
Thereafter					0
Total amortization of debt and principal repayments					933,000,000
Credit Facility 2012
Corporate Indebtedness
Letters of credit					73,100,000
Ability to borrow under line of credit facility					532,900,000
Mortgage and Other Loans Payable
Scheduled Amortization
2016					3,566,000
2017					7,411,000
2018					7,799,000
2019					8,207,000
2020					8,637,000
Thereafter					22,786,000
Total amortization of debt					58,406,000
Principal Repayments
2016					253,424,000
2017					0
2018					0
2019					0
2020					0
Thereafter					441,594,000
Total principal repayments					695,018,000
Senior Unsecured Notes
Scheduled amortization and principal repayments
2016					255,308,000
2017					0
2018					250,000,000
2019					0
2020					250,000,000
Thereafter					300,000,000
Total amortization of debt and principal repayments					1,055,308,000
Senior Unsecured Notes
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					1,055,308,000
Accreted Balance					1,055,106,000	955,001,000
Senior Unsecured Notes | 6.00% senior unsecured notes maturing on March 31, 2016
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					255,308,000
Accreted Balance					$ 255,296,000	255,250,000
Coupon Rate (as a percent)					6.00%
Effective Rate (as a percent)					6.00%
Term (in Years)					10 years
Senior Unsecured Notes | 5.00% senior unsecured notes maturing on August 15, 2018
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					$ 250,000,000
Accreted Balance					$ 249,810,000	249,744,000
Coupon Rate (as a percent)					5.00%
Effective Rate (as a percent)					5.00%
Term (in Years)					7 years
Senior Unsecured Notes | 7.75% senior unsecured notes maturing on March 15, 2020
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					$ 250,000,000
Accreted Balance					$ 250,000,000	250,000,000
Coupon Rate (as a percent)					7.75%
Effective Rate (as a percent)					7.75%
Term (in Years)					10 years
Senior Unsecured Notes | 4.50% senior unsecured notes maturing on December 01, 2022
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					$ 200,000,000
Accreted Balance					$ 200,000,000	200,000,000
Coupon Rate (as a percent)					4.50%
Effective Rate (as a percent)					4.50%
Term (in Years)					10 years
Senior Unsecured Notes | 4.27% senior unsecured notes maturing on December 17, 2025
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					$ 100,000,000
Accreted Balance					$ 100,000,000	0
Coupon Rate (as a percent)					4.27%
Effective Rate (as a percent)					4.27%
Term (in Years)					10 years
Senior Unsecured Notes | 4.00% senior unsecured notes redeemed in April 2015
Debt disclosures by scheduled maturity date
Unpaid Principal Balance					$ 0
Accreted Balance					$ 0	$ 7,000
Senior Unsecured Notes | 3.00% exchangeable senior notes due 2017
Debt disclosures by scheduled maturity date
Coupon Rate (as a percent)					3.00%
LIBOR | Revolving credit facility
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.25%
LIBOR | Revolving credit facility | Minimum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					0.875%
LIBOR | Revolving credit facility | Maximum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.55%
LIBOR | Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.40%
LIBOR | Term loan | Minimum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					0.95%
LIBOR | Term loan | Maximum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.90%