ACR MLG READ BACK WRONG ALT AND WAS IN CTLED FLT TOWARD TERRAIN.

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SLC CENTER CLRED MY FLT TO WHAT I THOUGHT WAS 11000' MSL AND TO CROSS THE FFU VOR AT THAT ALT. I READ BACK THAT CLRNC TO SLC CENTER WHILE MY F/O WAS BUSY ON ANOTHER RADIO GETTING ATIS FOR LNDG. I READ BACK: 'I BELIEVE YOU SAID 11000' AT THE FAIRFIELD VOR.' THERE WAS NO RESPONSE FROM ATC. WHILE DSNDING THROUGH 13800' MSL; ATC ASKED WHAT OUR ALT WAS AND INFORMED US THAT OUR CLRED ALT WAS 17000' MSL! ATC THEN CLRED US TO MAINTAIN 13000' MSL. FLT CONTINUED UNEVENTFULLY TO LNDG. SEVERAL FACTORS PROBABLY CONTRIBUTED TO THIS EVENT: MY MISUNDERSTANDING OF THE ORIGINAL CLRNC TO 17000'; MY INDIRECT QUESTIONING OF ATC CONCERNING THE CLRED ALT; ATC FAILED TO CORRECT MY READBACK; THIS WAS NOT A 2-M COCKPIT CREW IN THIS SITUATION SINCE I WAS BOTH FLYING AND TALKING TO ATC--F/O WAS NOT IN LOOP BECAUSE OF OTHER COCKPIT DUTIES NOT IN LOOP; MY NEGLECTING TO VERIFY MCA IN MOUNTAINOUS TERRAIN BY REF TO AREA CHART--NUMEROUS MTN PEAKS ABOVE 10000' MSL IN AREA!! SUPPLEMENTAL INFO FROM ACN 82314: AFTER LNDG WE CONTACTED THE SECTOR SUPVR ON THE PHONE TO INQUIRE ABOUT TERRAIN CLRNC AS NOBODY HAD NOTICED A 3200' DEVIATION. HE INFORMED US THE COMPUTER WILL ONLY ALERT THE CTLR IF THERE IS A TFC CONFLICT (THERE WAS NOBODY OUT THERE BUT US) OR IF WE HAD CONTINUED DOWN BELOW THE MINIMUM TERRAIN CLRNC ALT FOR THE AREA 10000-11000'. THE CTLR JUST HAPPENED TO NOTICE OUR MODE C.