The US Treasury spent its entire $6 billion ceiling in Thursday's bond buyback, retiring long-dated debt well ahead of maturity. Demand was anything but thin. Investors put forward $46.4 billion of bonds for the operation, and the government took the maximum, spread across just two of the 41 eligible issues, according to the official results. The two selected bonds pay low coupons and do not mature until 2041 and 2042. Treasury paid roughly $67 and $76 for every $100 of face value, which means retiring $6 billion of principal required about $4.47 billion in cash.  [...]