millions | 2013 | 2012 | 2011 | |||||||||
Aggregate fair value of shares granted based on weighted average fair value at date of grant | $ | 7 | $ | 9 | $ | 23 | ||||||
Aggregate fair value of shares vesting during period | 14 | 10 | 5 | |||||||||
Aggregate fair value of shares forfeited during period | 4 | 4 | 8 | |||||||||