The Collaborative International Dictionary of English v.0.48:

Thoroughness \Thor"ough*ness\, n.
   The quality or state of being thorough; completeness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thoroughness
    n 1: conscientiousness in performing all aspects of a task

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "thoroughness":
   assiduity, assiduousness, calculation, canniness, care,
   careful consideration, carefulness, caution, cautiousness,
   circumspection, completeness, comprehensiveness, deliberate stages,
   deliberateness, deliberation, diligence, discretion, entireness,
   entirety, exhaustiveness, gingerliness, guardedness, hedge,
   hedging, heed, heedfulness, hesitation, inclusiveness,
   industriousness, industry, intactness, integrality, integrity,
   judiciousness, mindfulness, omnipresence, pains, painstaking,
   painstakingness, pawkiness, pervasiveness, prior consultation,
   prudence, prudentialness, regardfulness, safeness, safety first,
   sedulousness, slowness to act, solicitude, solidarity, solidity,
   tentativeness, thoroughgoingness, totality, ubiquity,
   uncommunicativeness, unity, universality, unprecipitateness,
   wholeness