Corporate Indebtedness (Senior Unsecured Notes)(Details)	6 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 06, 2010 USD ($) $ / shares
Debt disclosures by scheduled maturity date
Accreted Balance	$ 1,180,217,000	$ 1,179,521,000
Senior unsecured notes
Debt disclosures by scheduled maturity date
Unpaid Principal Balance	1,094,967,000
Accreted Balance	1,091,332,000	1,133,957,000
Deferred financing costs, net	(4,396,000)	(5,642,000)
Accreted Balance, Net of Deferred Financing Costs	1,086,936,000	1,128,315,000
Senior unsecured notes | 3.00% Senior unsecured notes maturing on October 15, 2017
Debt disclosures by scheduled maturity date
Unpaid Principal Balance	294,967,000
Accreted Balance	$ 291,416,000	334,077,000
Coupon Rate (as a percent)	3.00%
Effective rate (as a percent)	3.00%
Initial Term (in Years)	7 years
Premium on sale price to calculate exchange price of notes (as a percent)			30.00%
Exchange price (in dollars per share) | $ / shares			$ 85.81
Adjusted exchange rate for the debentures (in shares)	12.8093
Principal amount of debentures, basis for conversion	$ 1,000
Amount of convertible debt recorded in equity			$ 78,300,000
Unamortized discount	3,600,000
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	$ 249,916,000	249,880,000
Coupon Rate (as a percent)	5.00%
Effective rate (as a percent)	5.00%
Initial 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%
Initial Term (in Years)	10 years
Senior unsecured notes | 4.50% Senior unsecured notes maturing on December 1, 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%
Initial 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	$ 100,000,000
Coupon Rate (as a percent)	4.27%
Effective rate (as a percent)	4.27%
Initial Term (in Years)	10 years