The following table summarizes the aggregate workers’ compensation reserve activity (in thousands):

	Three Months Ended
	March 31,
	2020			2019
Beginning balance
Workers' compensation claims liabilities	$	438,986		$	413,397
Add: claims expense accrual
Current period		38,492			40,385
Prior periods		(813	)		(1,700	)
		37,679			38,685
Less: claim payments related to
Current period		1,550			1,459
Prior periods		29,677			27,716
		31,227			29,175

Change in claims incurred in excess of retention limits		(128	)		(35	)

Ending balance
Workers' compensation claims liabilities	$	445,310		$	422,872
Incurred but not reported (IBNR)	$	282,497		$	268,291

Ratio of IBNR to workers' compensation claims liabilities		63	%		63	%

The Company is a self-insured employer with respect to workers' compensation coverage for all of its employees (including employees co-employed through our client service agreements) working in Colorado, Maryland and Oregon. In the state of Washington, state law allows only the Company's staffing services and internal management employees to be covered under the Company's self-insured workers' compensation program. The Company also operates a wholly owned, fully licensed insurance company, Ecole, which provides workers’ compensation coverage to the Company’s employees working in Arizona and Utah.

For all other clients, the Company obtains policies from Chubb Limited (“Chubb”) through an arrangement known as a fronted program, which provides a licensed, admitted insurance carrier to issue policies on behalf of the Company. Chubb assumes credit risk should the Company be unable to satisfy its indemnification obligations.

Through various insurance arrangements, the Company retains risk of loss up to the first $5.0 million per occurrence, except in Maryland and Colorado, where our retention per occurrence is $1.0 million and $2.0 million, respectively.

The fronted program with Chubb requires that collateral be advanced at the inception of the policy term. To partially satisfy these collateral requirements, the Company provided a surety bond of $15.0 million and a letter of credit of $63.7 million from its principal bank, Wells Fargo Bank, National Association (the “Bank”).

In addition, the Company makes monthly collateral payments into trust accounts (the “Chubb trust accounts”) for the fronted program. The balance in the Chubb trust accounts was $411.1 million and $393.5 million at March 31, 2020 and December 31, 2019, respectively. The Chubb trust accounts’ balances are included as a component of the current and long-term restricted cash and investments on the Company’s condensed consolidated balance sheets.

The states of California, Maryland, Oregon, Washington, Colorado and Delaware required us to maintain collateral totaling $76.1 million at March 31, 2020 and December 31, 2019 to cover potential workers’ compensation claims losses related to the Company’s current and former status as a self-insured employer. At March 31, 2020, the Company provided surety bonds and standby letters of credit totaling $76.1 million, including a California requirement of $55.6 million.

The Company provided a total of $445.3 million and $439.0 million at March 31, 2020 and December 31, 2019, respectively, as an estimated future liability for unsettled workers' compensation claims liabilities. Of this amount, $3.2 million and $3.3 million at March 31, 2020 and December 31, 2019, respectively, represent case reserves incurred in excess of the Company’s retention. The accrual for costs incurred in excess of retention limits is offset by a receivable from excess insurance carriers of $3.2 million and $3.3 million at March 31, 2020 and December 31, 2019, respectively, included in other assets on the condensed consolidated balance sheets.