4. Investments

The Company's trading securities consist of the following:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value (Net Carrying Amount)
December 31, 2017
Trading equity securities	$	4,964	$	394	$	31	$	5,327
Trading debt securities		10,971		58		179		10,850
Total	$	15,935	$	452	$	210	$	16,177
December 31, 2016
Trading equity securities	$	3,980	$	40	$	118	$	3,902
Trading debt securities		11,312		23		248		11,087
Total	$	15,292	$	63	$	366	$	14,989

Trading equity investments are valued at their estimated fair value based on their quoted market prices and trading debt securities are valued based upon a mix of observable market prices and model driven prices derived from a matrix of observable market prices for assets with similar characteristics obtained from a nationally recognized third-party pricing service. Additionally, a significant portion of the trading equity securities are in equity money market and mutual funds and also comprise a portion of the Company's liability under its SERP. See Note 12, Pension and Retirement Plans, for additional information on these investments and the SERP.

Trading debt securities are comprised mainly of marketable debt securities held by Astec Insurance. Astec Insurance has an investment strategy that focuses on providing regular and predictable interest income from a diversified portfolio of high-quality fixed income securities.

Net unrealized gains or losses incurred on investments still held as of the end of each reporting period amounted to losses of $319, $107 and $429 in 2017, 2016 and 2015, respectively.