The following table summarizes components of selected balance sheet accounts as of the dates presented:



	As of December 31,
	2012		2011
Other current assets:
Tubular inventory and well materials	$	4,033	$	14,251
Prepaid expenses	562		699
	$	4,595	$	14,950
Other assets:
Debt issuance costs	$	13,186	$	16,993
Assets of supplemental employee retirement plan (“SERP”) ¹	3,237		3,088
Other	1,511		51
	$	17,934	$	20,132
Accounts payable and accrued liabilities:
Trade accounts payable	$	37,835	$	30,186
Drilling costs	37,703		30,948
Royalties	14,390		15,235
Production and franchise taxes	2,874		3,495
Compensation - related ^{2, 3}	6,853		5,186
Interest	5,828		5,964
Preferred stock dividends	1,687		—
Other	4,485		3,490
	$	111,655	$	94,504
Other liabilities:
Firm transportation obligation	$	14,333	$	—
Asset retirement obligations	4,513		6,283
Defined benefit pension obligations ²	1,821		1,763
Postretirement health care benefit obligations ²	2,634		3,022
Deferred compensation - SERP obligation and other ¹	3,310		3,172
Other	2,290		1,647
	$	28,901	$	15,887

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¹ Represents the assets and liabilities of our nonqualified supplemental employee retirement savings plan. Assets of the plan are held in a Rabbi Trust. Shares of our common stock held by the Rabbi Trust are presented as treasury stock carried at cost.

² Includes the combined unfunded benefit obligations under our defined benefit pension and postretirement health care plans of $5.1 million and $5.4 million as of December 31, 2012 and 2011. The expense recognized with respect to these plans was $0.3 million, $0.4 million and $0.6 million for the years ended December 31, 2012, 2011 and 2010, respectively.

³ Includes employer matching obligations under our defined contribution retirement plan of $0.2 million and $0.3 million as of December 31, 2012 and 2011, respectively. The expense recognized with respect to this plan was $0.9 million, $1.2 million and $1.7 million for the years ended December 31, 2012, 2011 and 2010, respectively.