Quarterly Results (Summarized Quarterly Results Of Operations) (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
Interim Reporting [Line Items]
Interest income	$ 61,166	$ 63,617	$ 65,963	$ 65,883	$ 63,981	$ 62,949	$ 63,194	$ 53,254	$ 256,629	$ 243,378	$ 199,778
Interest expense	(22,859)	(20,061)	(18,638)	(16,290)	(17,743)	(17,930)	(15,894)	(14,513)	(77,848)	(66,080)	(56,251)
Net interest income (expense)	38,307	43,556	47,325	49,593	46,238	45,019	47,300	38,741	178,781	177,298	143,527
Other revenue (expense)	(3,288)	(3,583)	(4,055)	(4,488)	(4,335)	(4,080)	(4,824)	(4,114)	(15,414)	(17,353)	(16,890)
Equity in earnings of unconsolidated affiliates	65	64	65	65	65	64	65	65	259	259	259
Net income	$ 35,084	$ 40,037	$ 43,335	$ 45,170	$ 41,968	$ 41,003	$ 42,541	$ 34,692	$ 163,626	$ 160,204	$ 126,896
Basic net income per common share	$ 0.31	$ 0.35	$ 0.40	$ 0.45					$ 1.50	$ 1.76	$ 1.53
Basic and diluted net income per common share					$ 0.43	$ 0.43	$ 0.48	$ 0.41
Diluted net income per common share	$ 0.31	$ 0.35	$ 0.40	$ 0.44					$ 1.50	$ 1.75	$ 1.52