Earnings and Dividends Per Share - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended										1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Senior convertible notes due 2040 [Member]	Dec. 31, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2010 Stock Options [Member]	Dec. 31, 2012 Forward equity [Member]	Dec. 31, 2011 Forward equity [Member]	Mar. 31, 2011 Warrants [Member]	Dec. 31, 2011 Warrants [Member]
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Basic average shares outstanding not include restricted shares	2.4	1.9	1.9
Options to purchase common shares not included in computation of diluted EPS						2.6	2.7	3.2
Senior convertible notes Convertible to common shares price					$ 15.76
Conversion price per share greater than applicable trading day					125.00%
Warrants to purchase common shares			10.0	10.0
Exercise of warrants for common stock shares											10.0
Warrants to purchase common shares included in the computation of diluted EPS												10.0
Forward Equity Agreement Entered, not included in the computation of diluted EPS									19.0	9.5