Corporate Indebtedness - Senior Unsecured Notes (Details) - USD ($)	1 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt disclosures by scheduled maturity date
Accreted Balance		$ 1,180,506,000	$ 1,179,521,000
Senior unsecured notes
Debt disclosures by scheduled maturity date
Unpaid Principal Balance		1,069,000,000
Accreted Balance		1,068,562,000	1,133,957,000
Deferred financing costs, net		(4,018,000)	(5,642,000)
Accreted Balance, net of deferred financing costs		1,064,544,000	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		269,000,000
Accreted Balance		$ 268,628,000	334,077,000
Coupon Rate (as a percent)		3.00%
Effective rate (as a percent)		3.00%
Initial Term (in Years)		7 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		$ 249,934,000	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
Subsequent Event | Senior unsecured notes | 3.00% Senior unsecured notes maturing on October 15, 2017
Debt disclosures by scheduled maturity date
Repayments of debt	$ 350,800,000
Subsequent Event | Senior unsecured notes | 3.25 Percent Senior Unsecured Notes Due October 2022
Debt disclosures by scheduled maturity date
Coupon Rate (as a percent)	3.25%
Face amount of loan	$ 500,000,000.0
Subsequent Event | Senior Unsecured Bonds | 4.50% Senior Unsecured Bonds Due December 2022
Debt disclosures by scheduled maturity date
Coupon Rate (as a percent)	4.50%
Effective rate (as a percent)	3.298%
Face amount of loan	$ 100,000,000
Redemption price, percentage	105.334%