original mortgage amount rather than the amortized loan balance. Net premiums written may be in excess of net premiums earned due to single premium policies. For single premiums, we receive a single premium payment at origination, which is recorded as
the distribution of claims over the life of a book. The average age of our insurance portfolio is young with 81% of our IIF as of December 31, 2017 having been originated since January 1, 2015. As a result, based on historical industry performance, we expect the number of defaults and claims we experience, as well as our provision for losses and loss adjustment expenses, to increase as our portfolio seasons. See "-Mortgage Insurance Earnings and Cash Flow Cycle" below.
The following is a summary of our IIF at December 31, 2017 by vintage:
Other income includes fees earned for information technology and customer support services provided to Triad, contract underwriting revenues and changes in the fair value of the insurance and certain reinsurance policies issued by Essent Re under the ACIS program. The increase in other income for the year ended December 31, 2016 compared to the same period in 2015 was primarily due to an increase in favorable net valuation adjustments associated with ACIS contracts formerly accounted for as insurance rather than as derivatives and an increase in contract underwriting revenue, partially offset by a decrease in Triad service fees associated with a reduction in the number of Triad's mortgage insurance policies in force. The fees earned from Triad will continue to decrease over time as Triad's existing policies are cancelled.
Effective December 31, 2015, Fannie Mae and Freddie Mac, at the direction of the FHFA, implemented new coordinated Private Mortgage Insurer Eligibility Requirements, which we refer to as the "PMIERs." The PMIERs represent the standards by which private mortgage insurers are eligible to provide mortgage insurance on loans owned or guaranteed by Fannie Mae and Freddie Mac. The PMIERs include financial strength requirements incorporating a risk-based framework that require approved insurers to have a sufficient level of liquid assets from which to pay claims. This risk-based framework provides that an insurer must hold a substantially higher level of required assets for insured loans that are in default compared to a performing loan. The PMIERs also include enhanced operational performance expectations and define remedial actions that apply should an approved insurer fail to comply with these requirements. As of December 31, 2017, Essent Guaranty, Inc., our GSE-approved mortgage insurance company, was in compliance with the PMIERs. As of December 31, 2017, Essent Guaranty, Inc.'s Available Assets were $1.59 billion and its Minimum Required Assets were $1.54 billion based on our interpretation of the PMIERs.
We lease office space in Pennsylvania, North Carolina, California and Bermuda under leases accounted for as operating leases. A portion of the space leased in North Carolina has been subleased to Triad; minimum lease payments shown above have not been reduced by minimum sublease rental income of $0.1 million due in 2018 under the non-cancelable sublease.
The Company's management is responsible for these consolidated financial statements, for maintaining effective internal control over financial reporting, and for its assessment of the effectiveness of internal control over financial reporting, included in Management’s Report on Internal Control Over Financial Reporting appearing under item 9A. Our responsibility is to express opinions on the Company’s consolidated financial statements and on the Company's internal control over financial reporting based on our audits. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) ("PCAOB") and are required to be independent with respect to the Company in accordance with the U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.
The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.
We measure the cost of employee services received in exchange for awards of equity instruments at the grant date of the award using a fair value based method. Prior to our initial public offering, we estimated the fair value of each nonvested share grant on the date of grant based on management's best estimate using methods further described in Note 10 of our consolidated financial statements. Subsequent to our initial public offering, fair value is determined on the date of grant based on quoted market prices. We recognize compensation expense on nonvested shares over the vesting period of the award. We adopted ASU 2016-09 Improvements to Employee Share-Based Payment Accounting (Topic 718) on January 1, 2017 and recorded a charge of $0.1 million to retained earnings as of that date representing a cumulative-effect adjustment associated with our election to recognize forfeitures as they occur rather than on an estimated basis. In connection with the adoption of this ASU, beginning in 2017, excess tax benefits and tax deficiencies associated with share-based payments are recognized as income tax expense or benefit in the income statement and treated as discrete items in the reporting period. Through December 31, 2016, excess tax benefits were recognized in additional paid-in-capital.
Effective December 31, 2015, Fannie Mae and Freddie Mac, at the direction of the Federal Housing Finance Agency, implemented new coordinated Private Mortgage Insurer Eligibility Requirements, which we refer to as the "PMIERs." The PMIERs represent the standards by which private mortgage insurers are eligible to provide mortgage insurance on loans owned or guaranteed by Fannie Mae and Freddie Mac. The PMIERs include financial strength requirements incorporating a risk-based framework that require approved insurers to have a sufficient level of liquid assets from which to pay claims. The PMIERs also include enhanced operational performance expectations and define remedial actions that apply should an approved insurer fail to comply with these requirements. As of December 31, 2017, Essent Guaranty, our GSE-approved mortgage insurance company, was in compliance with the PMIERs.