Capital Actions (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Capital Actions
Repurchase amount	(125)
Repurchase Shares Authorized	100
November 6, 2012 ASR
Capital Actions
Redemption of common shares		7,710,761
Repurchase amount		125
Incremental common shares	657,914
Redemption date		Nov. 09, 2012
Forward contract redemption date	Feb. 12, 2013
December 14, 2012 ASR
Capital Actions
Redemption of common shares		6,267,410
Repurchase amount		100
Incremental common shares	127,760
Repurchase Shares Authorized		100,000,000
Redemption date		Dec. 19, 2012
Forward contract redemption date	Feb. 27, 2013
January 28, 2013 ASR
Capital Actions
Redemption of common shares	6,953,028
Repurchase amount	125
Incremental common shares	849,037
Redemption date	Jan. 31, 2013
Forward contract redemption date	Apr. 05, 2013