Notes and Bonds Payable	3 Months Ended
Notes and Bonds Payable	Mar. 31, 2013
Debt Disclosure [Abstract]
Notes and Bonds Payable	Notes and Bonds Payable On February 15, 2013, the Company entered into an amendment that extended the original maturity date of the unsecured credit facility ("Unsecured Credit Facility") from October 14, 2015 to April 14, 2017. The amendment also provides the Company with two six-month extension options that could extend the maturity date to April 14, 2018. Each option is subject to an extension fee of 0.075% of the aggregate commitments. Amounts outstanding under the Unsecured Credit Facility bear interest at LIBOR plus an applicable margin rate. The margin rate, which depends on the Company's credit ratings, ranges from 0.95% to 1.75% (currently at 1.4%). In addition, the Company pays a facility fee per annum on the aggregate amount of commitments ranging from 0.15% to 0.35% (currently at 0.30%). In connection with the amendment, the Company paid up-front fees to the lenders of approximately $2.7 million, included in Other assets, which will be amortized over the term of the facility. The Company wrote-off certain unamortized deferred financing costs of the original facility of approximately $0.3 million upon execution of the amendment. As of March 31, 2013, the Company had no borrowings outstanding under the Unsecured Credit Facility and had a remaining borrowing capacity of approximately $700.0 million. In February 2013, the Company repaid in full a mortgage note payable in the amount of $14.9 million bearing interest at a rate of 6.55% per year. On March 26, 2013, the Company issued $250.0 million of unsecured senior notes due 2023 (the "Senior Notes due 2023") that bear interest at 3.75%, payable semi-annually in arrears on April 15 and October 15, commencing October 15, 2013, and mature on April 15, 2023, unless redeemed earlier by the Company. The Senior Notes due 2023 were issued at a discount of approximately $2.1 million, which yielded a 3.85% interest rate per annum upon issuance. The Company incurred $2.1 million in debt issuance costs that are included in Other assets which will be amortized to maturity. The Senior Notes due 2023 have various financial covenant provisions that are required to be met on a quarterly and annual basis. The Company is in compliance with the covenant provisions at March 31, 2013. On April 18, 2013, the Company redeemed its unsecured senior notes due 2014 at a redemption price equal to an aggregate of $277.3 million, consisting of outstanding principal of $264.7 million, accrued interest as of the redemption date of $0.7 million, and a "make-whole" amount of approximately $11.9 million for the early extinguishment of debt. The unaccreted discount on these notes of $0.2 million and deferred financing costs of $0.2 million were written off upon redemption. In the second quarter of 2013, the Company will recognize a loss on early extinguishment of debt of approximately $12.3 million related to this redemption.

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

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