[Official Guidance]

Text of IRS Final Regs on Employer Shared Responsibility Rules under the ACA
Transition guidance is described in Section XV of the Preamble, which begins at page 105 of the 227-page pre-publication PDF version. (Internal Revenue Service)  

Obama Administration Delays Part of Employer Mandate Again
"Small businesses with fewer than 50 workers have always been exempt from the new coverage requirements but the law originally required all other businesses to start covering their workers for face penalties beginning Jan. 1, 2014. The White House last July pushed that start date to 2015, in response to concerted pressure from the business community. The new policy Treasury announced gives the mid-size businesses -- those with 50 to 99 workers -- another year to adapt to the changing health care marketplace." (Politico)  

White House Delays Health Insurance Mandate for Medium-Sized Employers Until 2016
"The Obama administration announced Monday it would give medium-sized employers an extra year, until 2016, before they must offer health insurance to their full-time workers. Firms with at least 100 employees will have to start offering this coverage in 2015 ... [but they] can avoid a fine by offering coverage to 70 percent of their full-time employees in 2015 and 95 percent starting in 2016. Under an earlier proposal, employers with at least 50 employees would have been required to offer insurance, beginning 2015, to 95 percent of those who work 30 hours or more a week, along with their dependents." (The Washington Post; subscription may be required)