I have been assuming that gains in electricity generation by natural gas were the reason for loss in generation by coal. This is not quite true. In 2016 coal generation was down by about 750 million kilowatt-hours (kWh) from its 2006-2008 average, a 38 percent drop. About two-thirds of the drop was due to natural gas and about one-third to renewables.
Source EIA Electric Data Browser, accessed 21 June 2017