From the Document: "Since the early 1960s the United States has maintained a 'triad' of strategic nuclear delivery vehicles. These include long-range land-based intercontinental ballistic missiles (ICBMs), long-range submarine-launched ballistic missiles (SLBMs), and long-range heavy bombers equipped to carry nuclear-armed cruise missiles and nuclear-armed gravity bombs. The number of nuclear warheads carried on these delivery vehicles peaked in the late 1980s, at around 14,000 warheads. It has been declining ever since, both as the United States complies with limits in U.S.-Russian arms control agreements and as it has changed requirements after the Cold War. At the present time, the United States is reducing its forces to comply with the New START Treaty [Strategic Arms Reduction Treaty], which entered into force in early 2011."
CRS In Focus, IF10519
Congressional Research Service: https://crsreports.congress.gov/