9. 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 nine month periods ended September 30, 2017 and 2016 (in thousands):


	For the Three Months					For the Nine Months
	Ended September 30,					Ended September 30,
	2017			2016		2017			2016
Interest costs charged to expense	$	21,936		$	19,598	$	65,164		$	42,780
Amortization of deferred financing costs		919			818		2,718			2,981
Accretion of debt discount on Convertible Notes		4,945			4,432		14,438			12,940
Amortization of debt premium on Senior Secured Notes		(215	)		—		(566	)		—

Interest expense		27,585			24,848		81,754			58,701
Interest costs capitalized to property and equipment		9			34		15			187
Interest costs capitalized to software		193			279		804			961

Total interest costs	$	27,787		$	25,161	$	82,573		$	59,849