10. Senior Unsecured Notes and Secured Debt

We have $4,429,992,000 of senior unsecured notes with annual stated interest rates ranging from 3.00% to 8.00%. The carrying amounts of the senior unsecured notes represent the par value of $4,464,930,000 adjusted for any unamortized premiums or discounts and other basis adjustments related to hedging the debt with derivative instruments. See Note 11 for further discussion regarding derivative instruments.

During the three months ended December 31, 2006, we issued $345,000,000 of 4.75% senior unsecured convertible notes due December 2026. The notes are convertible, in certain circumstances, into cash and, if applicable, shares of common stock at an initial conversion rate of 20.8833 shares per $1,000 principal amount of notes, which represents an initial conversion price of approximately $47.89 per share. In general, upon conversion, the holder of each note would receive, in respect of the conversion value of such note, cash up to the principal amount of such note and common stock for the note’s conversion value in excess of such principal amount. In addition, on each of December 1, 2011, December 1, 2016 and December 1, 2021, holders may require us to purchase all or a portion of their notes at a purchase price in cash equal to 100% of the principal amount of the notes to be purchased, plus any accrued and unpaid interest. During the three months ended March 31, 2009, we extinguished $5,000,000 of these notes and recognized a gain of $446,000. During the six months ended June 30, 2010, we extinguished $214,412,000 of these notes, recognized a loss of $8,837,000 and paid $18,552,000 to reacquire the equity component of convertible debt. As of June 30, 2011, we had $125,588,000 of these notes outstanding.

In July 2007, we issued $400,000,000 of 4.75% senior unsecured convertible notes due July 2027. The notes are convertible, in certain circumstances, into cash and, if applicable, shares of our common stock at an initial conversion rate of 20.0000 shares per $1,000 principal amount of notes, which represents an initial conversion price of approximately $50.00 per share. In general, upon conversion, the holder of each note would receive, in respect of the conversion value of such note, cash up to the principal amount of such note and common stock for the note’s conversion value in excess of such principal amount. In addition, on each of July 15, 2012, July 15, 2017 and July 15, 2022, holders may require us to purchase all or a portion of their notes at a purchase price in cash equal to 100% of the principal amount of the notes to be purchased, plus any accrued and unpaid interest. During the three months ended March 31, 2009, we extinguished $5,000,000 of these notes and recognized a gain of $594,000. During the six months ended June 30, 2010, we extinguished $226,914,000 of these notes, recognized a loss of $16,235,000 and paid $21,062,000 to reacquire the equity component of convertible debt. As of June 30, 2011, we had $168,086,000 of these notes outstanding.

During the year ended December 31, 2010, we issued $494,403,000 of 3.00% senior unsecured convertible notes due December 2029. The notes are convertible, in certain circumstances, into cash and, if applicable, shares of common stock at an initial conversion rate of 19.5064 shares per $1,000 principal amount of notes, which represents an initial conversion price of approximately $51.27 per share. In general, upon conversion, the holder of each note would receive, in respect of the conversion value of such note, cash up to the principal amount of such note and common stock for the note’s conversion value in excess of such principal amount. In addition, on each of December 1, 2014, December 1, 2019 and December 1, 2024, holders may require us to purchase all or a portion of their notes at a purchase price in cash equal to 100% of the principal amount of the notes to be purchased, plus any accrued and unpaid interest. In connection with this issuance, we recognized $29,925,000 of equity component of convertible debt.

During the three months ended June 30, 2010, we issued $450,000,000 of 6.125% senior unsecured notes due 2020, generating net proceeds of $446,328,000. During the three months ended September 30, 2010, we issued $450,000,000 of 4.70% senior unsecured notes due 2017, generating net proceeds of $445,768,000. During the three months ended March 31, 2011, we issued $400,000,000 of 3.625% senior unsecured notes due 2016, $600,000,000 of 5.25% senior unsecured notes due 2022 and $400,000,000 of 6.50% senior unsecured notes due 2041, generating net proceeds of $1,381,086,000.

We have secured debt totaling $1,889,873,000, collateralized by owned properties, with annual interest rates ranging from 4.60% to 10.00%. The carrying amounts of the secured debt represent the par value of $1,869,129,000 adjusted for any unamortized fair value adjustments on loan assumptions. The carrying values of the properties securing the debt totaled $3,215,570,000 at June 30, 2011. During the six months ended June 30, 2010, we assumed $106,140,000 of first mortgage loans principal with an average rate of 7.35% secured by 17 properties. During the six months ended June 30, 2011, we assumed $689,876,000 of first mortgage loans principal with an average rate of 5.388% secured by 35 properties. During the six months ended June 30, 2011, we issued $58,470,000 of first mortgage loans principal with an average rate of 5.78% secured by 32 properties.

Our debt agreements contain various covenants, restrictions and events of default. Certain agreements require us to maintain certain financial ratios and minimum net worth and impose certain limits on our ability to incur indebtedness, create liens and make investments or acquisitions. As of June 30, 2011, we were in compliance with all of the covenants under our debt agreements.

At June 30, 2011, the annual principal payments due on these debt obligations are as follows (in thousands):