Summarized Quarterly Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Total interest income	$ 771,249	$ 697,160	$ 712,936	$ 737,217	$ 756,661	$ 761,265	$ 886,324	$ 854,895	$ 2,918,562	$ 3,259,145	$ 3,579,618
Less: Total interest expense	137,393	145,476	164,255	177,590	185,491	181,893	166,443	133,345	624,714	667,172	480,326
Net interest income	633,856	551,684	548,681	559,627	571,170	579,372	719,881	721,550	2,293,848	2,591,973	3,099,292
Total other income (loss)	452,944	(299,925)	1,154,755	368,370	163,282	(277,689)	(734,828)	264,633	1,676,144	(584,602)	(2,459,576)
Less: Total general and administrative expenses	56,294	58,744	65,131	51,912	40,084	63,004	64,556	67,915
Income (loss) before income taxes and income from equity method investment in affiliate	1,030,506	193,015	1,638,305	876,085	694,368	238,679	(79,503)	918,268	3,737,911	1,771,812	402,372
Less: Income taxes	1,757	557	92	5,807	(6,127)	13,921	11,656	16,462	8,213	35,912	59,051
Net income (loss)	1,028,749	192,458	1,638,213	870,278	700,495	224,758	(91,159)	901,806	3,729,698	1,735,900	344,461
Less: Dividends on preferred stock	17,992	17,992	17,992	17,992	19,717	9,367	6,508	3,938	71,968	39,530	16,854
Net income (loss) available (related) to common shareholders	$ 1,010,757	$ 174,466	$ 1,620,221	$ 852,286	$ 680,778	$ 215,391	$ (97,667)	$ 897,868	$ 3,657,730	$ 1,696,370	$ 327,607
Net income (loss) available (related) per share to common shareholders:
Basic	$ 1.07	$ 0.18	$ 1.71	$ 0.90	$ 0.70	$ 0.22	$ (0.10)	$ 0.92	$ 3.86	$ 1.74	$ 0.37
Diluted	$ 1.03	$ 0.18	$ 1.64	$ 0.87	$ 0.68	$ 0.22	$ (0.10)	$ 0.89	$ 3.74	$ 1.71	$ 0.37