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

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2020			2019			2020			2019
Beginning balance
Workers' compensation claims liabilities	$	336,504		$	430,736		$	438,986		$	413,397
Add: claims expense accrual
Current period		35,138			37,404			106,359			118,934
Prior periods		(2,977	)		(5,593	)		(5,163	)		(10,245	)
		32,161			31,811			101,196			108,689
Less: claim payments related to
Current period		7,046			8,609			13,123			14,213
Prior periods		15,640			23,815			181,170			77,619
		22,686			32,424			194,293			91,832

Change in claims incurred in excess of retention limits		1,369			(77	)		1,459			(208	)

Ending balance
Workers' compensation claims liabilities	$	347,348		$	430,046		$	347,348		$	430,046
Incurred but not reported (IBNR)	$	203,595		$	274,344		$	203,595		$	274,344

Ratio of IBNR to workers' compensation claims liabilities		59	%		64	%		59	%		64	%

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 $3.0 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 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 $277.4 million and $393.5 million at September 30, 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 $59.7 million and $76.1 million at September 30, 2020 and December 31, 2019, respectively, to cover potential workers’ compensation claims losses related to the Company’s current and former status as a self-insured employer. At September 30, 2020, the Company provided surety bonds and standby letters of credit totaling $59.7 million, including a California requirement of $39.2 million.

On June 29, 2020, the Company entered into a loss portfolio transfer agreement to remove all outstanding workers’ compensation claims obligations for claims incurred under its fronted insurance program between February 1, 2014 and December 31, 2017. This transaction reduced the Company’s outstanding workers’ compensation liabilities and Chubb trust account balances by $115.7 million.

As of September 30, 2020, the Company recorded a receivable from Chubb of $0.7 million related to claim payments made on Chubb’s behalf for claim obligations transferred to Chubb as part of the loss portfolio transfer agreement. The amount is included in prepaid expenses and other on the condensed consolidated balance sheets.

The Company provided a total of $347.3 million and $439.0 million at September 30, 2020 and December 31, 2019, respectively, as an estimated future liability for unsettled workers' compensation claims liabilities. Of this amount, $4.8 million and $3.3 million at September 30, 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 $4.8 million and $3.3 million at September 30, 2020 and December 31, 2019, respectively, included on other assets on the condensed consolidated balance sheets.