10. Interest Costs

We capitalize a portion of our interest on funds borrowed during the active construction period of major capital projects. Capitalized interest is added to the cost of the underlying assets and amortized over the useful lives of the assets.

The following is a summary of our interest costs for the three and six month periods ended June 30, 2018 and 2017 (in thousands):


	For the Three Months						For the Six Months
	Ended June 30,						Ended June 30,
	2018			2017			2018			2017
Interest costs charged to expense	$	24,928		$	21,689		$	49,908		$	43,228
Amortization of deferred financing costs		1,048			903			2,083			1,799
Accretion of debt discount on Convertible Notes		5,368			4,812			10,592			9,493
Amortization of debt premium on Senior Secured Notes		(703	)		(178	)		(1,388	)		(351	)

Interest expense		30,641			27,226			61,195			54,169
Interest costs capitalized to property and equipment		3			2			15			6
Interest costs capitalized to software		75			253			107			611

Total interest costs	$	30,719		$	27,481		$	61,317		$	54,786