Air carrier Captain reported a severe turbulence encounter during cruise. Altitude was stabilized and the flight continued.

Narrative

We were cruising at FL 370 in IMC and encountered severe turbulence. The aircraft's altitude initially went down to 36;700; then up to 37;670; then down to 36;860 before stabilizing at 37;000 again. The FO was PF and was focused on manually overriding the Autothrottles so we didn't overspeed the aircraft. The autopilot did not disengage; so he left it on. After we returned to FL 370; I reported the turbulence and altitude deviation to ZZZ Center. They copied the info and told us to maintain FL 370; but no further communication with us on the subject. We both had our seatbelts buckled and no one was injured. The aircraft did not sustain any damage either.