Net Income Per Common Share (Components Of Computation Of Basic And Diluted Net Income Per Common Share) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income	$ 35,084	$ 40,037	$ 43,335	$ 45,170	$ 41,968	$ 41,003	$ 42,541	$ 34,692	$ 163,626	$ 160,204	$ 126,896
Series A and B preferred share dividends									(21,021)	(20,369)	(20,233)
Unvested stock award participation in earnings									(372)	(579)	(594)
Numerator for basic net income per common share									142,233	139,256	106,069
Weighted average common shares outstanding									95,115	79,784	69,948
Average unvested stock awards outstanding									(522)	(468)	(396)
Denominator for basic net income per common share									94,593	79,316	69,552
Basic net income per common share	$ 0.31	$ 0.35	$ 0.40	$ 0.45					$ 1.50	$ 1.76	$ 1.53
Net income	35,084	40,037	43,335	45,170	41,968	41,003	42,541	34,692	163,626	160,204	126,896
Dividends on antidilutive convertible preferred shares									(20,723)	(20,071)	(19,932)
Unvested stock award participation in earnings									(372)	(579)	(594)
Numerator for diluted net income per common share									$ 142,531	$ 139,554	$ 106,370
Weighted average common shares outstanding									95,115	79,784	69,948
Average unvested stock awards outstanding									(522)	(468)	(396)
Net effect of dilutive stock and option awards									111	71	39
Net effect of dilutive convertible preferred shares									308	309	310
Denominator for diluted net income per common share									95,012	79,696	69,901
Diluted net income per common share	$ 0.31	$ 0.35	$ 0.40	$ 0.44					$ 1.50	$ 1.75	$ 1.52