8. Notes Payable

A. General

Our senior unsecured notes and bonds consisted of the following, sorted by maturity date (dollars in millions):

	December 31,			December 31,
	2015			2014
5.5% notes, issued in November 2003 and due in November 2015	$	-		$	150
5.95% notes, issued in September 2006 and due in September 2016	275			275
5.375% notes, issued in September 2005 and due in September 2017	175			175
2.0% notes, issued in October 2012 and due in January 2018	350			350
6.75% notes, issued in September 2007 and due in August 2019	550			550
5.75% notes, issued in June 2010 and due in January 2021	250			250
3.25% notes, issued in October 2012 and due in October 2022	450			450
4.65% notes, issued in July 2013 and due in August 2023	750			750
3.875% notes, issued in June 2014 and due in July 2024	350			350
4.125% notes, issued in September 2014 and due in October 2026	250			250
5.875% bonds, $100 issued in March 2005 and $150 issued in
June 2011, both due in March 2035	250			250

Total principal amount	3,650			3,800
Unamortized original issuance discounts	(13		)	(15		)

	$	3,637		$	3,785

The following table summarizes the maturity of our notes and bonds payable as of December 31, 2015, excluding unamortized original issuance discounts (dollars in millions):

Year of Maturity	Principal
2016	$	275
2017	175
2018	350
2019	550
2020	-
Thereafter	2,300

Totals	$	3,650

As of December 31, 2015, the weighted average interest rate on our notes and bonds payable was 4.7% and the weighted average remaining years until maturity was 6.5 years.

Interest incurred on all of the notes and bonds was $179.5 million for 2015, $166.5 million for 2014 and $138.9 million for 2013. The interest rate on each of these notes and bonds is fixed.

Our outstanding notes and bonds are unsecured; accordingly, we have not pledged any assets as collateral for these or any other obligations. Interest on all of the senior note and bond obligations is paid semiannually.

All of these notes and bonds contain various covenants, including: (i) a limitation on incurrence of any debt which would cause our debt to total adjusted assets ratio to exceed 60%; (ii) a limitation on incurrence of any secured debt which would cause our secured debt to total adjusted assets ratio to exceed 40%; (iii) a limitation on incurrence of any debt which would cause our debt service coverage ratio to be less than 1.5 times; and (iv) the maintenance at all times of total unencumbered assets not less than 150% of our outstanding unsecured debt. At December 31, 2015, we remain in compliance with these covenants.

B. Note Issuances

In September 2014, we issued $250 million of 4.125% senior unsecured notes due October 2026, or the 2026 Notes. The price to the investors for the 2026 Notes was 99.499% of the principal amount for an effective yield of 4.178% per annum. A portion of the total net proceeds of approximately $246.4 million from this offering were used to repay all outstanding borrowings under our credit facility, and the remaining proceeds were used for other general corporate purposes, including additional property acquisitions.

In June 2014, we issued $350 million of 3.875% senior unsecured notes due July 2024, or the 2024 Notes. The price to the investors for the 2024 Notes was 99.956% of the principal amount for an effective yield of 3.88% per annum. The total net proceeds of approximately $346.7 million from these offerings were used to repay a portion of the outstanding borrowings under our previous credit facility.

C. Note Repayment

In November 2015, we repaid $150 million of outstanding 5.5% notes, plus accrued and unpaid interest, using proceeds from our October 2015 common stock offering and our new credit facility.