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

	Years Ended December 31,
	2019			2018			2017
Beginning balance
Workers' compensation claims liabilities	$	413,397		$	363,517		$	312,537
Add: claims expense accrual
Current period		161,691			162,525			154,091
Prior periods		(13,355	)		(3,846	)		5,159
		148,336			158,679			159,250
Less: claim payments related to
Current period		24,414			23,444			19,537
Prior periods		98,445			85,603			82,573
		122,859			109,047			102,110
Change in claims incurred in excess of retention limits		112			248			(6,160	)
Ending balance
Workers' compensation claims liabilities	$	438,986		$	413,397		$	363,517
Incurred but not reported (IBNR)	$	285,191		$	260,529		$	202,227

Ratio of IBNR to workers' compensation claims liabilities		65	%		63	%		56	%

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 $393.5 million and $451.0 million at December 31, 2019 and December 31, 2018, 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 consolidated balance sheets.

The states of California, Maryland, Oregon, Washington, Colorado and Delaware required us to maintain collateral totaling $76.1 million and $85.2 million at December 31, 2019 and 2018, respectively, to cover potential workers’ compensation claims losses related to the Company’s current and former status as a self-insured employer. At December 31, 2019, 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 $439.0 million and $413.4 million at December 31, 2019 and 2018, respectively, as an estimated future liability for unsettled workers' compensation claims liabilities. Of this amount, $3.3 million and $3.2 million at December 31, 2019 and 2018, 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 $3.3 million and $3.2 million at December 31, 2019 and 2018, respectively, included in other assets on the consolidated balance sheets.