RESTRICTED CASH AND INVESTMENTS

Restricted cash and investments consist principally of collateral that has been provided or pledged to insurance carriers for workers’ compensation and state workers’ compensation programs. Our insurance carriers and certain state workers’ compensation programs require us to collateralize a portion of our workers’ compensation obligation. The collateral typically takes the form of cash and cash equivalents and highly rated investment grade securities, primarily in debt and asset-backed securities. The majority of our collateral obligations are held in a trust at the Bank of New York Mellon (“Trust”).

The following is a summary of the carrying value of our restricted cash and investments:



(in thousands)	September 30, 2018		December 31, 2017
Cash collateral held by insurance carriers	$	24,054	$	22,926
Cash and cash equivalents held in Trust	28,494		16,113
Investments held in Trust	162,519		171,752
Deferred compensation mutual funds	25,410		22,428
Other restricted cash and cash equivalents	157		6,012
Total restricted cash and investments	$	240,634	$	239,231

The amortized cost and estimated fair value of our held-to-maturity investments held in trust, aggregated by investment category are as follows:



	September 30, 2018
(in thousands)	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Municipal debt securities	$	77,211	$	124	$	(1,101	)	$	76,234
Corporate debt securities	81,414		1		(1,221		)	80,194
Agency mortgage-backed securities	2,895		3		(52		)	2,846
U.S. government and agency securities	999		—		(35		)	964
Total held-to-maturity investments	$	162,519	$	128	$	(2,409	)	$	160,238



	December 31, 2017
(in thousands)	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
Municipal debt securities	$	82,770	$	974	$	(378	)	$	83,366
Corporate debt securities	83,916		309		(434		)	83,791
Agency mortgage-backed securities	4,066		22		(26		)	4,062
U.S. government and agency securities	1,000		19		—			1,019
Total held-to-maturity investments	$	171,752	$	1,324	$	(838	)	$	172,238

The estimated fair value and gross unrealized losses of all investments classified as held-to-maturity, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position as of September 30, 2018 and December 31, 2017, were as follows:



	September 30, 2018
	Less than 12 months					12 months or more					Total
(in thousands)	Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses
Municipal debt securities	$	49,727	$	(515	)	$	11,531	$	(586	)	$	61,258	$	(1,101	)
Corporate debt securities	52,187		(689		)	26,020		(532		)	78,207		(1,221		)
Agency mortgage-backed securities	1,323		(20		)	873		(32		)	2,196		(52		)
U.S. government and agency securities	964		(35		)	—		—			964		(35		)
Total held-to-maturity investments	$	104,201	$	(1,259	)	$	38,424	$	(1,150	)	$	142,625	$	(2,409	)



	December 31, 2017
	Less than 12 months					12 months or more					Total
(in thousands)	Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses
Municipal debt securities	$	23,078	$	(124	)	$	9,631	$	(254	)	$	32,709	$	(378	)
Corporate debt securities	48,952		(311		)	10,081		(123		)	59,033		(434		)
Agency mortgage-backed securities	1,362		(10		)	888		(16		)	2,250		(26		)
Total held-to-maturity investments	$	73,392	$	(445	)	$	20,600	$	(393	)	$	93,992	$	(838	)

The total number of held-to-maturity securities in an unrealized loss position as of September 30, 2018 and December 31, 2017 were 124 and 83, respectively. The unrealized losses were the result of interest rate increases. Since the decline in estimated fair value is attributable to changes in interest rates and not credit quality, and the company has the intent and ability to hold these debt securities until recovery of amortized cost or until maturity, we do not consider these investments other than temporarily impaired.

The amortized cost and fair value by contractual maturity of our held-to-maturity investments are as follows:



	September 30, 2018
(in thousands)	Amortized cost		Fair value
Due in one year or less	$	28,294	$	28,133
Due after one year through five years	87,267		86,225
Due after five years through ten years	46,958		45,880
Total held-to-maturity investments	$	162,519	$	160,238

Actual maturities may differ from contractual maturities because the issuers of certain debt securities have the right to call or prepay their obligations without penalty. We have no significant concentrations of counterparties in our held-to-maturity investment portfolio.