To obtain a 64-bit SteamID given a Text SteamID in the form "STEAM_X:Y:Z" and assuming the accounts instance and type are equal 1 (usual case):

Steam64 = 0;
Instance = 1;
Type = 1;
Steam64 |= Y;
Steam64 |= ((Z & 0x7fffffff) << 1);
Steam64 |= ((Instance & 0xfffff) << 32);
Steam64 |= ((Type & 0xf) << 52);
Steam64 |= ((X & 0xff) << 56);
return Steam64;