Summarized Quarterly Results (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
Total interest income	$ 756,661	$ 761,265	$ 886,324	$ 854,895	$ 847,700	$ 930,802	$ 957,068	$ 844,048	$ 3,259,145	$ 3,579,618	$ 2,683,134
Less: Total interest expense	185,491	181,893	166,443	133,345	130,133	121,417	113,320	115,456	667,172	480,326	428,225
Net interest income	571,170	579,372	719,881	721,550	717,567	809,385	843,748	728,592	2,591,973	3,099,292	2,254,909
Total other income (loss)	163,282	(277,689)	(734,828)	264,633	(191,614)	(1,650,587)	(652,940)	35,565	(584,602)	(2,459,576)	(783,293)
Less: Total general and administrative expenses	40,084	63,004	64,556	67,915	63,094	65,194	57,229	51,827	235,559	237,344	171,847
Income (loss) before income taxes and income from equity method investment in affiliate	694,368	238,679	(79,503)	918,268	462,859	(906,396)	133,579	712,330	1,771,812	402,372	1,299,769
Less: Income taxes	(6,127)	13,921	11,656	16,462	17,297	15,417	12,762	13,575	35,912	59,051	35,434
Income from equity method investment in affiliate								1,140		1,140	2,945
Net income (loss)	700,495	224,758	(91,159)	901,806	445,562	(921,813)	120,817	699,895	1,735,900	344,461	1,267,280
Less: Dividends on preferred stock	19,717	9,367	6,508	3,938	4,148	4,172	4,267	4,267	39,530	16,854	18,033
Net income (loss) available (related) to common shareholders	$ 680,778	$ 215,391	$ (97,667)	$ 897,868	$ 441,414	$ (925,985)	$ 116,550	$ 695,628	$ 1,696,370	$ 327,607	$ 1,249,247
Net income (loss) available (related) per share to common shareholders:
Basic	$ 0.70	$ 0.22	$ (0.10)	$ 0.92	$ 0.46	$ (0.98)	$ 0.14	$ 0.92	$ 1.74	$ 0.37	$ 2.12
Diluted	$ 0.68	$ 0.22	$ (0.10)	$ 0.89	$ 0.44	$ (0.98)	$ 0.14	$ 0.89	$ 1.71	$ 0.37	$ 2.04