Acquisition costs are comprised of commissions, brokerage fees, insurance taxes and other acquisition-related costs such as profit commissions and amortization of insurance-related intangible assets, and are reduced for ceding commission income received on our ceded reinsurance. Commissions and brokerage fees are usually calculated as a percentage of premiums and depend on the market and line of business. Acquisition costs are reported after (1) deducting commissions received on ceded reinsurance, (2) deducting the part of deferred acquisition costs relating to the successful acquisition of new and renewal insurance and reinsurance contracts and (3) including the amortization of previously deferred acquisition costs.
As of December 31, 2016, our changes in quota share premium estimates have been adjustments of (0.5)%, (0.1)% and 2.5% for the 2015, 2014 and 2013 treaty years, respectively. For the 2016 treaty year, if we assume the average change of premium estimates for the past three years was 1%, then it is reasonably likely that our gross premiums written in our Reinsurance segment would increase or decrease by $2.1 million over the next three years. There would also be a related increase or decrease in loss and loss expenses and acquisition costs due to the increase or decrease in gross premiums written.
There have been no material changes to any of our valuation techniques from those used as of December 31, 2015. Based on all reasonably available information received, we believe the prices that were obtained from inactive markets were orderly transactions and therefore, reflected the current price a market participant would pay for the asset. Since fair valuing a financial instrument is an estimate of what a willing buyer would pay for our asset if we sold it, we will not know the ultimate value of our financial instruments until they are sold. We believe the valuation techniques utilized provide us with the best estimate of the price that would be received to sell our assets in an orderly transaction between participants at the measurement date.
Acquisition costs increased by $80.3 million, or 27.2%, for the year ended December 31, 2015 compared to the year ended December 31, 2014. The increase in acquisition costs was primarily due to higher acquisition costs in our North American Insurance and Global Markets Insurance segments, including the additional amortization of the insurance-related intangible asset recorded from the acquisitions of the RSA operations. Acquisition costs as a percentage of net premiums earned were 15.1% for the year ended December 31, 2015 compared to 13.5% for 2014. The increase in the acquisition cost ratio was primarily driven by lower ceding commission income due to restructuring some of our underlying reinsurance programs in our North American Insurance and Global Markets Insurance segments and higher ceding commissions paid to cedents in our Reinsurance segment.
Other insurance-related income and expense. The other insurance-related income of $4.3 million for the year ended December 31, 2016 represents the recovery on single-trigger ILW derivatives related to catastrophe losses that occurred in the current year and the other insurance-related expense of $3.4 million related to unrealized losses recognized on single-trigger ILW derivatives. During the year ended December 31, 2015, we incurred $1.0 million of unrealized losses recognized on single-trigger ILW derivatives.
Premiums. Gross premiums written decreased by $137.2 million, or 14.6%, for the year ended December 31, 2015 compared to 2014. The decrease was primarily due to lower premiums written across each line of business. The decrease for each line of business was driven by non-renewal of business due to poor terms and conditions, cedents retaining more of their
The payment of dividends from Holdings’ Bermuda-domiciled operating subsidiary is, under certain circumstances, limited under Bermuda law, which requires our Bermuda operating subsidiary to maintain certain measures of solvency and liquidity.
Cash flows provided by or used in investing activities consist primarily of payments for investments acquired, proceeds on the sale of investments and changes in restricted cash. The increase in cash flows used in investing activities to cash flows provided by investing activities was due to higher net sales of investments during the current period compared to the prior period, due to the maturity and sale of securities to repay the $500.0 million aggregate principal amount of 7.50% senior notes