Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Jan. 31, 2014	Dec. 31, 2013
Financial assets:
U.S. Treasury securities			$ 1,117,915
Agency mortgage-backed securities	75,350,388		70,388,949
Agency debentures	2,408,259		2,969,885
Investment in affiliates	137,647		139,447
Interest rate swaps	380,995		705,769
Other derivatives	40,105		146,725
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned	513,510	260,700	2,527,668	[1]
Securitized debt of consolidated VIE	260,700
Interest rate swaps	1,278,661		1,197,346
Other derivatives	6,045		55,518
Level 1
Financial assets:
U.S. Treasury securities			1,117,915
Investment in affiliates	137,647		139,447
Other derivatives			3,487
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Other derivatives			439
Level 2
Financial assets:
Agency mortgage-backed securities	75,350,388		70,388,949
Agency debentures	2,408,259		2,969,885
Other derivatives	40,105		143,238
Financial liabilities:
Other derivatives	6,045		55,079
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	924,197		552,436
Reverse repurchase agreements	444,375		100,000
Securities borrowed	513,500		2,582,893
U.S. Treasury securities			1,117,915
Investment in affiliates	137,647		139,447
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned	513,510		2,527,668
Convertible Senior Notes	883,946		870,199
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	75,350,388		70,388,949
Agency debentures	2,408,259		2,969,885
Corporate debt	145,912		118,362
Interest rate swaps	340,890		559,044
Financial liabilities:
Securitized debt of consolidated VIE	262,105
Mortgages payable	19,240		19,240
Interest rate swaps	1,272,616		1,141,828
Estimate of Fair Value, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,649,239		1,581,836
Financial liabilities:
Participation sold	13,882		14,050
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	40,105		146,725
Financial liabilities:
Repurchase agreements	64,856,432		62,134,133
Other derivatives	6,045		55,518
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	924,197		552,436
Reverse repurchase agreements	444,375		100,000
Securities borrowed	513,500		2,582,893
U.S. Treasury securities			1,117,915
Investment in affiliates	137,647		139,447
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned	513,510		2,527,668
Convertible Senior Notes	827,486		825,262
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	75,350,388		70,388,949
Agency debentures	2,408,259		2,969,885
Corporate debt	145,394		117,687
Interest rate swaps	340,890		559,044
Financial liabilities:
Securitized debt of consolidated VIE	260,700
Mortgages payable	19,317		19,332
Interest rate swaps	1,272,616		1,141,828
Carrying (Reported) Amount, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,640,206		1,583,969
Financial liabilities:
Participation sold	13,963		14,065
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	40,105		146,725
Financial liabilities:
Repurchase agreements	64,543,949		61,781,001
Other derivatives	$ 6,045		$ 55,518

[1] Derived from the audited consolidated financial statements at December 31, 2013.