Summary of Performance Based Shares and Units by Year of Grant (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares and units granted, net of forfeitures	4,899,503
Weighted average grant date fair value (1)	$ 46.51	[1]
Shares and units nonvested	2,568,588
2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of grant	2008
Shares and units granted, net of forfeitures	2,269,011
Weighted average grant date fair value (1)	$ 39.66	[1]
Shares and units nonvested	884,914
2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of grant	2009
Shares and units granted, net of forfeitures	904,167
Weighted average grant date fair value (1)	$ 44.06	[1]
Shares and units nonvested	415,917
2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of grant	2010
Shares and units granted, net of forfeitures	737,937
Weighted average grant date fair value (1)	$ 63.28	[1]
Shares and units nonvested	435,383
2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of grant	2011
Shares and units granted, net of forfeitures	650,058
Weighted average grant date fair value (1)	$ 53.73	[1]
Shares and units nonvested	494,044
2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Year of grant	2012
Shares and units granted, net of forfeitures	338,330
Weighted average grant date fair value (1)	$ 48.65	[1]
Shares and units nonvested	338,330

[1] Amount shown is the weighted average grant date fair value of shares and units granted, net of forfeitures.