Condensed Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Interest income	$ 1,866,664	$ 1,913,794	$ 1,634,612
OTTI loss on securities	(18,124)	(1,971)	(96,533)
Gain on debt repurchases		3,008	10,054
Other income	35,453	33,783	24,564
Operating expenses	574,683	546,246	384,003
Income before income tax benefit and equity in (overdistributed) undistributed earnings of subsidiaries	734,577	837,471	593,149
Income tax benefit	(254,540)	(296,454)	(194,503)
Net income	480,037	541,017	398,646
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest income	1,064	969	1,837
Dividends received from subsidiaries	555,000	335,000	300,000
OTTI loss on securities			(13,200)
Gain on debt repurchases			8,792
Other income	753	767	881
Gross income	556,817	336,736	298,310
Operating expenses	42,185	39,394	41,134
Income before income tax benefit and equity in (overdistributed) undistributed earnings of subsidiaries	514,632	297,342	257,176
Income tax benefit	16,445	17,127	20,511
Income before equity in (overdistributed) undistributed earnings of subsidiaries	531,077	314,469	277,687
Equity in (overdistributed )undistributed earnings of subsidiaries	(51,040)	226,548	120,959
Net income	$ 480,037	$ 541,017	$ 398,646