balance is immediately recognized as earned premium revenue. Substantially all of our single premium policies in force as of December 31, 2018 were non-refundable. Premiums collected on annual policies are recognized as net premiums earned on a straight-line basis over the year of coverage. For the year ended December 31, 2018, monthly and single premium policies comprised 84.7% and 15.3% of our NIW, respectively.
Net premiums earned increased in the year ended December 31, 2017 by 25% compared to the year ended December 31, 2016 primarily due to the increase in our average IIF from $73.0 billion in 2016 to $96.0 billion in 2017, partially offset by the decrease in average premium rate from 0.57% for the year ended December 31, 2016 to 0.53% for the year ended December 31, 2017. The decrease in the average premium rate during the year ended December 31, 2017 is due primarily to the decrease in premium earned on the cancellation of non-refundable single premium policies.
Radnor Re 2018-1 Ltd. is a special purpose variable interest entity that is not consolidated in our consolidated financial statements because we do not have the unilateral power to direct those activities that are significant to its economic performance. As of December 31, 2018, our estimated off-balance sheet maximum exposure to loss from Radnor Re was $18.3 million, representing the estimated net present value of investment earnings on the assets in the reinsurance trust. See Note 5 to our consolidated financial statements for additional information.
The amortized cost and fair value of investments available for sale at December 31, 2018, by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. Because most U.S. agency mortgage-backed securities, residential and commercial mortgage securities and asset-backed securities provide for periodic payments throughout their lives, they are listed below in separate categories.
In the event that the compounded annual book value per share growth falls between the performance levels shown above, the nonvested common shares earned will be determined on a straight-line basis between the respective levels shown. The compounded annual book value per share growth for each of the 2013, 2014, 2015 and 2016 performance-based grants exceeded the maximum performance level and have vested or will vest at 100%.
The nonvested performance-based share awards are considered contingently issuable for purposes of the EPS calculation. Based on the compounded annual book value per share growth as of December 31, 2018, 2017 and 2016, 100% of the performance-based share awards would be issuable under the terms of the arrangements at each date if December 31 was the end of the contingency period.
Essent Re's statutory capital and surplus was $798.5 million and $662.6 million as of December 31, 2018 and 2017, respectively, and statutory net income was $140.2 million and $96.9 million, respectively. Statutory capital and surplus and net income determined in accordance with statutory accounting practices were not significantly different than the amounts determined under GAAP.