INVESTMENTS

We classify all of our short-term investments as available-for-sale. We carry these at fair value and report unrealized gains and losses, net of taxes, in Accumulated other comprehensive loss, which is a separate component of shareholders’ equity in our Condensed Consolidated Balance Sheets. These unrealized gains and losses are also reflected in our Condensed Consolidated Statements of Comprehensive Income and Condensed Consolidated Statements of Shareholders’ Equity. We determine cost, gains, and losses using the specific identification method.

Investments consisted of the following as of September 30, 2018:



	Cost Basis		Unrealized Gains		Unrealized Losses		Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	50,290	$	—	$	—	$	50,290
Deferred compensation plan assets included in other assets	852		—		—		852
Total	$	51,142	$	—	$	—	$	51,142

Investments consisted of the following as of December 31, 2017:



	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	5,601	$	—	$	—		$	5,601
U.S. government agencies	7,501		—		(34		)	7,467
Corporate bonds and notes	63,880		—		(330		)	63,550
Subtotal	76,982		—		(364		)	76,618
Deferred compensation plan assets included in other assets	2,685		—		—			2,685
Total	$	79,667	$	—	$	(364	)	$	79,303

At September 30, 2018 and December 31, 2017, we classified $19.8 million and $12.4 million, respectively of our aggregate investments as long-term investments and recorded them in our Condensed Consolidated Balance Sheets as restricted cash and investments, as they are securing outstanding letters of credit with maturities beyond one year.

The following table presents the cost and fair value of our debt investments based on maturities as of:



	September 30, 2018				December 31, 2017
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	—	$	—	$	31,348	$	31,254
Due within two years	—		—		40,032		39,763
Total	$	—	$	—	$	71,380	$	71,017

The following table presents the average coupon rate percentage and the average days to maturity of our debt investments as of:



	September 30, 2018		December 31, 2017
	Average Coupon Rate (%)	Average Days To Maturity	Average Coupon Rate (%)	Average Days To Maturity
U.S. Government agencies	—	0	1.370	370
Corporate bonds and notes	—	0	1.766	392

The following table presents the fair value and unrealized losses related to our investments that have been in a continuous unrealized loss position for less than twelve months as of:



	September 30, 2018				December 31, 2017
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Thousands of dollars)
U.S. Government agencies	$	—	$	—	$	5,472	$	(28	)
Corporate bonds and notes	—		—		44,069		(271		)
Total	$	—	$	—	$	49,541	$	(299	)

The following table presents the fair value and unrealized losses related to our investments that have been in a continuous unrealized loss position for more than twelve months as of:



	September 30, 2018				December 31, 2017
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Thousands of dollars)
U.S. Government agencies	$	—	$	—	$	1,994	$	(6	)
Corporate bonds and notes	—		—		19,482		(59		)
Total	$	—	$	—	$	21,476	$	(65	)

From time to time over the periods covered in our financial statements included herein (and as illustrated in the foregoing tables), our investments have experienced net unrealized losses. We consider these declines in market value to be due to customary market fluctuations, and we typically do not plan to sell these investments prior to maturity. For these reasons, we do not consider any of our investments to be other than temporarily impaired at December 31, 2017. For additional information regarding our criteria for making these assessments, see Note 2 to the financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2017.