Buffet's crazy put options

Buffet has collected a $4.5 bn premium by selling 15-20 year put options on four stock indices (including S&P), stuck at the current level of market. Who are these geniuses who are so bearish on the market over a 15 year time-span that they paid a $4.5bn premium? Simply inflation would push up the level of markets - so the probablity of this event is low. But if the probability of this event is low, Buffet must have written this on a staggering huge notional amount - something like a trillion dollars. Is it Fidelity or Vanguard or one of these giant asset management firms who have bought these options - cant think of anyone else who has a trillion dollars to protect? Or am I reading this incorrectly? Why hasnt anyone else picked this up?