The results of our fully-insured dental HMO/IND, fully-insured dental PPO and self-insured dental segments are measured by gross profit. We do not measure the gross profit of our corporate, all other segment. We do not allocate investment and other income, insurance expenses, assets or liabilities to our segments because these measures are not used to analyze the segments. Our segments do not share overhead costs or assets. We do, however, measure the contributions of each of our fully-insured and self-insured segments to costs retained in our corporate, all other segment.
Our ratio of healthcare services expense to premium revenue (“loss ratio”) remained constant at 78.0% in 2015 and 2014. The fully-insured dental HMO/IND and fully-insured dental PPO segments together represent approximately 69.8% of our total dental business.
Self-insured healthcare services expense increased by approximately $1,914,000, or 8.3%, in 2015. An increase of 5.3% in self-insured member month volume in 2015 compared to 2014 resulted in an increase in self-insured healthcare services expense of approximately $1,225,000. Self-insured dental healthcare services expense increased by approximately $364,000 due to the fee schedule increases effective January 1, 2015. The remaining increase in self-insured claims expense of approximately $325,000 is due to an increase in dental service utilization level by our self-insured members in 2015 compared to 2014.
Corporate, all other premium revenue is primarily derived from the non-DCP dental indemnity product, dental PPO product and vision product underwritten by third party insurance carriers. In the aggregate, corporate, all other premium revenue increased by approximately $52,000, or 9.1%, to $624,000 in 2014 from $572,000 in 2013.
Most states rely on risk-based capital requirements, or RBC, to define the required levels of equity. RBC is a model developed by the NAIC to monitor an entity’s solvency. This calculation indicates recommended minimum levels of required capital and surplus and signals regulatory measures should actual surplus fall below these recommended levels. RBC has been adopted the states in which we currently do business. We file our annual statement and RBC reporting with the Ohio Department of Insurance and the NAIC. Dental Care Plus’s statutory annual statements for the year ended December 31, 2015 filed with the Ohio Department of Insurance reflected total adjusted capital in excess of Company Action Level RBC.
Unrealized losses at December 31, 2015 and 2014 on investments were generally due to higher current market yields relative to the yields of the investments at their amortized cost. Unrealized losses due to credit risk associated with the underlying collateral of the investments, if any, are not material. All investments with unrealized losses are reviewed quarterly to determine if any impairment is other than temporary, requiring a charge to earnings. The Company considers the severity of the loss on a security, duration of loss, duration of the investment, credit rating of the security, as well as payment performance and the Company’s intent or requirement to sell the investment before recovery when determining whether any impairment is other than temporary. During 2015, the Company had six other-than-temporarily impaired securities which were recorded as a realized loss of approximately $111,000. The Company had no other-than-temporary impairment charges for the years ended December 2014 and 2013. The Company had no investments in a material unrealized loss position for greater than one year as of December 31, 2015 and 2014.