Earnings Per Share - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Senior convertible notes due 2040 [Member]
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Senior convertible notes convertible to common shares price	15.65
Conversion price per share greater than applicable trading day	125.00%
Stock Options [Member]
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase common shares not included in computation of diluted EPS	2,800,000	2,900,000
Forward equity [Member]
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Forward Equity Agreement entered, not included in the computation of diluted EPS		18,975,000