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

	Years Ended December 31,
	2018			2017			2016
Beginning balance
Workers' compensation claims liabilities	$	363,517		$	312,537		$	255,675
Add: claims expense accrual
Current period		162,525			154,091			137,852
Prior periods		(3,846	)		5,159			(301	)
		158,679			159,250			137,551
Less: claim payments related to
Current period		23,444			19,537			20,180
Prior periods		85,603			82,573			69,626
		109,047			102,110			89,806
Add: change in claims incurred in excess of retention limits		248			(6,160	)		9,117
Ending balance
Workers' compensation claims liabilities	$	413,397		$	363,517		$	312,537
Incurred but not reported (IBNR)	$	260,529		$	202,227		$	158,169

Ratio of IBNR to workers' compensation claims liabilities		63	%		56	%		51	%

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 obtains policies from Chubb for all clients in California, Delaware, Virginia, Pennsylvania, North Carolina, New Jersey, West Virginia, Idaho, Nevada and the District of Columbia. The arrangement with Chubb, known as a fronted program, provides BBSI a licensed, admitted insurance carrier to issue policies on behalf of BBSI. The risk of loss up to the first $5.0 million per occurrence is retained by BBSI through various agreements. Chubb assumes credit risk should BBSI be unable to satisfy its indemnification obligations.

The Company’s wholly owned, fully licensed captive insurance company incorporated in Arizona, Associated Insurance Company for Excess (“AICE”), provides reinsurance coverage up to $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 Company maintains excess workers’ compensation insurance coverage with Chubb between $5.0 million and statutory limits per occurrence, except in Maryland, where coverage with Chubb is between $1.0 million and statutory limits per occurrence, and in Colorado, where the coverage with Chubb is between $2.0 million and statutory limits per occurrence.

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. The Company maintains additional reinsurance coverage for Ecole with Chubb Limited (“Chubb”), for losses above $5.0 million per occurrence.

The Company restructured its fronted program with Chubb effective July 1, 2018. The new agreement maintains retention levels of $5.0 million per occurrence but now requires that collateral be advanced at the inception of the policy term. To partially satisfy these additional collateral requirements, the Company provided a surety bond of $30.0 million and a letter of credit of $63.7 million from its principal bank, Wells Fargo Bank, National Association (the “Bank”).

As part of its fronted workers’ compensation insurance program with Chubb, the Company makes monthly payments into trust accounts (the “Chubb trust accounts”) to be used for the payment of future claims. The balance in the Chubb trust accounts was $451.0 million and $380.6 million at December 31, 2018 and December 31, 2017, 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 specified investment balances or other financial instruments totaling $85.2 million and $96.8 million at December 31, 2018 and 2017, 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, 2018, the Company provided surety bonds and standby letters of credit totaling $85.2 million, including a California requirement of $70.6 million.

The Company provided a total of $413.4 million and $363.5 million at December 31, 2018 and 2017, respectively, as an estimated future liability for unsettled workers' compensation claims liabilities. Of this amount, $3.2 million and $3.0 million at December 31, 2018 and 2017, 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 $3.0 million at December 31, 2018 and 2017, respectively, included in other assets on the consolidated balance sheets.