2. 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 (income), 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 March 31, 2016:


	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	17,877	$	—	$	—		$	17,877
Commercial paper		5,994		1		—			5,995
U.S. Government agencies		27,301		7		(6	)		27,302
Corporate bonds and notes		250,399		94		(192	)		250,301

Subtotal		301,571		102		(198	)		301,475
Deferred compensation plan assets included in other assets		2,334		—		—			2,334

Total	$	303,905	$	102	$	(198	)	$	303,809

Investments consisted of the following as of December 31, 2015:


	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	18,181	$	—	$	—		$	18,181
Commercial paper		5,986		—		(5	)		5,981
U.S. Government agencies		11,499		—		(30	)		11,469
Corporate bonds and notes		265,069		—		(841	)		264,228

Subtotal		300,735		—		(876	)		299,859
Deferred compensation plan assets included in other assets		2,294		—		—			2,294

Total	$	303,029	$	—	$	(876	)	$	302,153

At March 31, 2016 and December 31, 2015, we classified $15.3 million of our aggregate investments as long-term investments and recorded them in our Condensed Consolidated Balance Sheets as Restricted 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:


	March 31, 2016				December 31, 2015
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	148,590	$	148,516	$	152,444	$	152,212
Due within two years		135,104		135,082		130,110		129,466

Total	$	283,694	$	283,598	$	282,554	$	281,678

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


	March 31, 2016				December 31, 2015
	Average Coupon Rate (%)		Average Days To Maturity		Average Coupon Rate (%)		Average Days To Maturity
Commercial paper		0.553		63		0.553		154
U.S. Government agencies		0.985		602		0.865		599
Corporate bonds and notes		1.727		328		1.757		331

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:


	March 31, 2016					December 31, 2015
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Commercial paper	$	0	$	0		$	5,981	$	(5	)
U.S. Government agencies		11,996		(6	)		8,969		(30	)
Corporate bonds and notes		134,298		(150	)		232,347		(793	)

Total	$	146,294	$	(156	)	$	247,297	$	(828	)

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:


	March 31, 2016					December 31, 2015
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Corporate bonds and notes	$	47,783	$	(42	)	$	28,866	$	(48	)

Total	$	47,783	$	(42	)	$	28,866	$	(48	)

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 market conditions, and we 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 March 31, 2016 or December 31, 2015. We have also determined that we did not have any other-than-temporary impairments relating to credit losses on debt securities for the three months ended March 31, 2016. 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, 2015.