Practically speaking, a nuclear detonation in a major city is not going to happen, because for a long time, sensors have been used that detect any significant radiation source.
They are good enough to every now and then get a reading from a person who has recently had radioisotope medical treatment. And compared to that, a nuclear bomb would look like a volcano.
And realistically speaking, the best terrorists might come up with is called a “dirty bomb”, which is basically high grade nuclear waste, blown up with ordinary explosives. It is not a fission explosion. But this has serious problems.
To start with, the most dangerous isotopes (think different metals), are quite heavy. If you took powdered lead or gold, and blew it up in the air, the vast majority would fall down within a few yards. And these isotopes are heavier than that.
The lightest isotopes have short half-lives, which means from the time they are created, the clock is ticking, and with every half-life, half of the isotopes are no longer radioactive. So by the time they are smuggled into the US, it would be pretty inert.
So it is the middle isotopes, like Cesium, that are the biggest worry. But the governments of the world intensely monitor the production and storage of these, so getting your hands on some is somewhat harder than breaking into a major bank vault.
cargo shop parked 12 miles off shore can easily hold a lead cased nuke. the Navy does not stop and search them all.
Any anti missile systems could not respond fast enough to stop a missile launch from such a ship before it covered several miles. A simple time problem. Even if it detonated 12 miles off shore, the fallout would end DC as a viable place to exist.