CAPT OF AN LTT FAILED TO STOP HIS DSCNT AT ASSIGNED ALT CAUSING AN ALT OVERSHOOT.

Narrative

AS WE APCHED THE SIOUX CITY AREA; ZMP GAVE US PLT'S DISCRETION TO 6000 FT. WE WERE AT 15000 FT ON TOP OF AN OVCST LAYER. THE CAPT STAYED AT 15000 FT. THE CAPT (PF) WAS BUSY WITH NON FLT; NON JOB RELATED TASKS (FILLING OUT PERSONAL CALENDAR). I PROMPTED CAPT FOR BEGINNING OF IN- RANGE CHKLIST (HE SHOULD HAVE CALLED FOR IT AT THIS POINT). UPON COMPLETION OF CHKLIST WE WERE SWITCHED TO APCH WHO GAVE US PLT'S DISCRETION TO 3400 FT AND TO CALL THE FIELD FOR A VISUAL TO RWY 17. WE WERE APCHING 20 MI OR LESS TO THE FIELD AND HAD NOT BEGUN OUR DSCNT; AND HAD A GND SPD OF 250 KTS. I MENTIONED THE LAYER BELOW US TO GET THE CAPT TO BEGIN DOWN. HE REACHED UP AND SELECTED THE INDICATED AIRSPD MODE ON THE AUTOPLT BUT ONLY REDUCED TORQUE BY ABOUT 10 PERCENT. WE WERE APCHING THE FIELD TOO HIGH; TOO FAST. I WAS LOOKING FOR THE RWY AS WE CAME THROUGH A BROKEN LAYER AND TOLD THE TWR WE COULD NOT MAKE RWY 17 BUT COULD TAKE RWY 13. AT THIS POINT I LOOKED UP AND WE HAD GONE THROUGH THE PREVIOUSLY ASSIGNED ALT OF 3400 FT BY ALMOST 1000 FT. IN RETROSPECT HAD I BEEN MORE ASSERTIVE IN GETTING MY CAPT TO PAY ATTN TO HIS JOB; THE SIT COULD HAVE BEEN AVOIDED. HE SHOULD NOT HAVE BEEN INVOLVED WITH NON FLT RELATED TASKS IN A CRITICAL PHASE OF FLT.