Summarized Quarterly Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest income	$ 648,144	$ 644,640	$ 683,962	$ 655,901	$ 771,249	$ 697,160	$ 712,936	$ 737,217	$ 2,632,647	$ 2,918,562	$ 3,259,145
Interest expense	134,512	127,069	126,107	124,971	137,393	145,476	164,255	177,590	512,659	624,714	667,172
Net interest income	513,632	517,571	557,855	530,930	633,856	551,684	548,681	559,627	2,119,988	2,293,848	2,591,973
Total other income (loss)	(1,113,659)	(109,013)	(842,030)	(682,902)	452,944	(299,925)	1,154,755	368,370	(2,747,604)	1,676,144	(584,602)
Less: Total general and administrative expenses	58,454	51,317	52,189	47,378	56,294	58,744	65,131	51,912
Income (loss) before income taxes	(658,481)	357,241	(336,364)	(199,350)	1,030,506	193,015	1,638,305	876,085	(836,954)	3,737,911	1,771,812
Less: Income taxes	(209)	2,385	(852)	4,001	1,757	557	92	5,807	5,325	8,213	35,912
Net income (loss)	(658,272)	354,856	(335,512)	(203,351)	1,028,749	192,458	1,638,213	870,278	(842,083)	3,729,698	1,735,900
Less: Net income attributable to noncontrolling interest	(196)								196
Less: Dividends on preferred stock	17,992	17,992	17,992	17,992	17,992	17,992	17,992	17,992	71,968	71,968	39,530
Net income (loss) available (related) to common stockholders	$ (676,068)	$ 336,864	$ (353,504)	$ (221,343)	$ 1,010,757	$ 174,466	$ 1,620,221	$ 852,286	$ (914,051)	$ 3,657,730	$ 1,696,370
Net income (loss) available (related) per common stockholders:
Basic	$ (0.71)	$ 0.36	$ (0.37)	$ (0.23)	$ 1.07	$ 0.18	$ 1.71	$ 0.90	$ (0.96)	$ 3.86	$ 1.74
Diluted	$ (0.71)	$ 0.35	$ (0.37)	$ (0.23)	$ 1.03	$ 0.18	$ 1.64	$ 0.87	$ (0.96)	$ 3.74	$ 1.71