What are some good general updated document widths for a starting a web project?

I would be tickled if someone could shed light on optimal website design sizes for 2015. I am a graphic designer first, and each and every time I begin a new website I find I get stuck at the planning stage, trying to discern what page width to use to mock up my site. Currently I have experimented with 960px-1000px-1140px grids, but I would like to hear what other designers are doing, and where they see things going.