Membership in our commercial health plans decreased by 67,000 members, or 3%, at December 31, 2007 compared to December 31, 2006. This decrease was primarily attributable to the mix shift from large group to small group/individual enrollment resulting in a net loss of 26,000 commercial risk members and a 41,000 ASO member loss. This mix shift was predominantly seen in our California plan, which experienced a decline of 73,000 large group members primarily from a loss of two large accounts, partially offset by a net gain of 58,000 small group and individual members. Our Northeast plans experienced a decline of 37,000 members in the large group market, which was partially offset by a net gain of 16,000 members in our New York small group market. As a result of our targeted mix shift, our small group and individual enrollment comprised approximately 35% of our commercial risk enrollment, excluding ASO, at December 31, 2007, up from 31% at December 31, 2006.
Commercial premium revenues increased by $329.5 million, or 4%, for the year ended December 31, 2008 as compared to the same period in 2007. These increases were primarily attributable to our ongoing pricing discipline and premium rate increases, partially offset by membership declines.
Amortization and depreciation expense increased by $16.9 million for the year ended December 31, 2008 as compared to the same period in 2007 primarily due to property and equipment purchased during the year, the addition of new assets placed in production related to various information technology system projects and the amortization of intangible assets from the Guardian Transaction. See Note 2-Summary of Significant Accounting Policies to the consolidated financial statements.
By law, regulation and governmental policy, our health plan and insurance subsidiaries, which we refer to as our regulated subsidiaries, are required to maintain minimum levels of statutory net worth. The minimum statutory net worth requirements differ by state and are generally based on balances established by statute, a percentage of annualized premium revenue, a percentage of annualized health care costs, or risk-based capital (RBC) requirements. The RBC requirements are based on guidelines established by the National Association of Insurance Commissioners. The RBC formula, which calculates asset risk, underwriting risk, credit risk, business risk and other factors, generates the authorized control level (ACL), which represents the minimum amount of net worth believed to be required to support the regulated entity’s business. For states in which the RBC requirements have been adopted, the regulated entity typically must maintain the greater of the Company Action Level RBC, calculated as 200% of the ACL, or the minimum statutory net worth requirement calculated pursuant to pre-RBC guidelines. Because our regulated subsidiaries are also subject to their state regulators’ overall oversight authority, some of our subsidiaries are required to maintain minimum capital and surplus in excess of the RBC requirement, even though RBC has been adopted in their states of domicile. We generally manage our aggregate regulated subsidiary capital above 300% of ACL, although RBC standards are not yet applicable to all of our regulated subsidiaries. At December 31, 2008, we had sufficient capital to exceed this level.
(b) The FIN 48 obligations shown above represent uncertain tax positions expected to be paid within the reporting periods presented. In addition to the obligations shown above, approximately $55.9 million of unrecognized tax benefits have been recorded as a liability in accordance with FIN 48, and we are uncertain as to if or when such amounts may be settled or paid.
If we identify an indicator of impairment, we assess recoverability by comparing the carrying amount of the asset to the sum of the undiscounted cash flows expected to result from the use and the eventual disposal of the asset. An impairment loss is recognized when the carrying amount is not recoverable and is measured as the excess of carrying value over fair value. We recognized such an impairment of $47.9 million in 2008 representing impairments of investment securities and long-lived assets, including property and equipment.