6. Indebtedness

We have a $750,000 unsecured revolving credit facility that is available for general business purposes, including acquisitions. The maturity date of our revolving credit facility is September 7, 2015 and, subject to the payment of an extension fee and meeting certain other conditions, includes an option for us to extend the stated maturity date of our revolving credit facility by one year to September 7, 2016. In addition, our revolving credit facility includes a feature under which maximum borrowings may be increased to up to $1,500,000 in certain circumstances. Borrowings under our revolving credit facility bear interest at a rate of LIBOR plus a premium of 130 basis points. We also pay a facility fee of 30 basis points per annum on the total amount of lending commitments under our revolving credit facility. Both the interest rate premium and the facility fee are subject to adjustment based upon changes to our credit ratings. As of December 31, 2012, the interest rate payable on borrowings under our revolving credit facility was 1.51%, and the weighted average annual interest rate for borrowings under our revolving credit facility was 1.56%, 1.59% and 0.9% for the years ended December 31, 2012, December 31 2011, and December 31, 2010, respectively. As of December 31, 2012 we had $320,000 outstanding and $430,000 available under our revolving credit facility.

On March 12, 2012, we entered into a five year $400,000 unsecured term loan. Our term loan matures on March 13, 2017, and is prepayable without penalty at any time. In addition, our term loan includes a feature under which maximum borrowings may be increased to up to $500,000 in certain circumstances. Our term loan bears interest at a rate of LIBOR plus a premium, which was 145 basis points as of December 31, 2012. The interest rate premium is subject to adjustment based upon changes to our credit ratings. As of December 31, 2012, the interest rate for the amount outstanding under our term loan was 1.67% and the weighted average interest rate for the amount outstanding under our term loan was 1.70% for the period from March 12, 2012 to December 31, 2012.

Our credit facility agreement and our term loan agreement provide for acceleration of payment of all amounts due thereunder upon the occurrence and continuation of certain events of default, including a change of control of us and the termination of our business management agreement with RMR.

Our revolving credit facility agreement and our term loan agreement contain a number of covenants that restrict our ability to incur debt in excess of calculated amounts, restrict our ability to make distributions under certain circumstances and generally require us to maintain certain financial ratios. We believe we were in compliance with the terms and conditions of our revolving credit facility agreement and our term loan agreement at December 31, 2012.

On March 20, 2012, we repurchased at par plus accrued and unpaid interest $70,576 of our 3.8% convertible senior notes due 2027 which were tendered by the holders of these notes for repurchase by us.

On April 11, 2012, we redeemed at par all of our outstanding 6.85% senior notes due 2012 for $100,829 plus accrued and unpaid interest.

On August 16, 2012, we issued $500,000 of 5.00% senior notes due 2022 in a public offering for net proceeds of $487,946 (after underwriting and other offering expenses).

On September 10, 2012, we redeemed at par all of our outstanding 6.75% senior notes due 2013 for $287,000 plus accrued and unpaid interest.

As of December 31, 2012, $8,478 of our 3.8% convertible senior notes due 2027 remains outstanding. The convertible senior notes are convertible if certain conditions are met (including certain changes in control) into cash equal to the principal amount of the notes and, to the extent the market price of our common shares exceeds the initial exchange price of $50.50 per share, subject to adjustment, either cash or our common shares at our option with a value based on such excess amount. Holders of our convertible senior notes may require us to repurchase all or a portion of the notes on March 15, 2017, and March 15, 2022, or upon the occurrence of certain change in control events.

We separately account for the liability (debt) and equity (conversion option) components of our 3.8% convertible senior notes due 2027 to reflect the fair value of the liability component based on our non-convertible borrowing cost at the issuance date. We measured the fair value of the debt components of the notes at issuance based on an estimated effective interest rate of 6.06% and are amortizing the resulting discount as an increase to interest expense over the expected life of the debt (assuming holders of the notes exercised in full their option to require us to repurchase the notes on March 20, 2012).

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The net carrying amount of our 3.8% convertible senior notes due 2027 was $8,478 and $78,823 as of December 31, 2012 and December 31, 2011, respectively.

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The unamortized discount on the notes was zero and $231 as of December 31, 2012 and December 31, 2011, respectively. We amortized the discount through March 20, 2012, the first date on which the holders of our 3.8% convertible senior notes could require that we redeem them.

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Interest expense for the years ended December 31, 2012, 2011 and 2010 includes non-cash amortization of $270, $1,605 and $2,602, respectively.

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The amount allocated as the equity component of the convertible senior notes was $37,710 as of December 31, 2012 and is included in additional paid in capital in our consolidated balance sheets.

At December 31, 2012 and 2011, our indebtedness was as follows:

	As of December 31,

	2012			2011
Senior Notes, due 2012 at 6.85%	$	—		$	100,829
Senior Notes, due 2013 at 6.75%		—			287,000
Senior Notes, due 2014 at 7.875%		300,000			300,000
Senior Notes, due 2015 at 5.125%		280,000			280,000
Senior Notes, due 2016 at 6.3%		275,000			275,000
Senior Notes, due 2017 at 5.625%		300,000			300,000
Senior Notes, due 2018 at 6.7%		350,000			350,000
Senior Notes, due 2022 at 5.0%		500,000			—
Convertible Senior Notes, due 2027 at 3.8%		8,478			79,054
Unamortized discounts		(11,120	)		(5,169	)

Total unsecured senior notes		2,002,358			1,966,714
Unsecured revolving credit facility		320,000			149,000
Unsecured term loan		400,000			—

	$	2,722,358		$	2,115,714

All of our senior notes are prepayable at any time prior to their maturity date at par plus accrued interest plus a premium equal to a make whole amount, as defined, generally designed to preserve a stated yield to the noteholder. Interest on all of our senior notes is payable semi-annually in arrears.

The required principal payments due during the next five years and thereafter under all our outstanding debt at December 31, 2012, are as follows:


2013	$	—
2014		300,000
2015		600,000	⁽¹⁾
2016		275,000
2017		700,000	⁽²⁾
Thereafter		858,478	⁽³⁾

	$	2,733,478

(1): Includes the $320,000 outstanding on our $750,000 revolving credit facility at December 31, 2012.
(2): Includes our $400,000 unsecured term loan.
(3): Includes our $8,478 convertible senior notes due 2027. Holders of our convertible senior notes may require us to repurchase all or a portion of the notes on March 15, 2017 and March 15, 2022, or upon the occurrence of certain change in control events.