Estimated Fair Value for All Financial Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Jan. 31, 2014	Dec. 31, 2013
Financial assets:
U.S. Treasury securities			$ 1,117,915
Agency mortgage-backed securities	81,565,256		70,388,949
Agency debentures	1,368,350		2,969,885
Investment in affiliates	143,045		139,447
Interest rate swaps	80,724		705,769
Other derivatives	5,499		146,725
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned			2,527,668
Securitized debt of consolidated VIE	260,700	260,700
Interest rate swaps	1,616,313		1,197,346
Other derivatives	8,027		55,518
Level 1
Financial assets:
U.S. Treasury securities			1,117,915
Investment in affiliates	143,045		139,447
Other derivatives	117		3,487
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Other derivatives	3,769		439
Level 2
Financial assets:
Agency mortgage-backed securities	81,565,256		70,388,949
Agency debentures	1,368,350		2,969,885
Other derivatives	5,382		143,238
Financial liabilities:
Other derivatives	4,258		55,079
Estimate of Fair Value, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,741,244		552,436
Reverse repurchase agreements	100,000		100,000
Securities borrowed			2,582,893
U.S. Treasury securities			1,117,915
Investment in affiliates	143,045		139,447
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned			2,527,668
Convertible Senior Notes	863,470		870,199
Estimate of Fair Value, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	81,565,256		70,388,949
Agency debentures	1,368,350		2,969,885
Corporate debt	166,056		118,362
Interest rate swaps	75,225		559,044
Financial liabilities:
Securitized debt of consolidated VIE	262,061
Mortgages payable	146,611		19,240
Interest rate swaps	1,608,286		1,141,828
Estimate of Fair Value, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,528,444		1,581,836
Financial liabilities:
Participation sold	13,655		14,050
Estimate of Fair Value, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	5,499		146,725
Financial liabilities:
Repurchase agreements	71,587,222		62,134,133
Other derivatives	8,027		55,518
Carrying (Reported) Amount, Fair Value Disclosure | Level 1
Financial assets:
Cash and cash equivalents	1,741,244		552,436
Reverse repurchase agreements	100,000		100,000
Securities borrowed			2,582,893
U.S. Treasury securities			1,117,915
Investment in affiliates	143,045		139,447
Financial liabilities:
U.S. Treasury securities sold, not yet purchased			1,918,394
Securities loaned			2,527,668
Convertible Senior Notes	845,295		825,262
Carrying (Reported) Amount, Fair Value Disclosure | Level 2
Financial assets:
Agency mortgage-backed securities	81,565,256		70,388,949
Agency debentures	1,368,350		2,969,885
Corporate debt	166,464		117,687
Interest rate swaps	75,225		559,044
Financial liabilities:
Securitized debt of consolidated VIE	260,700
Mortgages payable	146,553		19,332
Interest rate swaps	1,608,286		1,141,828
Carrying (Reported) Amount, Fair Value Disclosure | Level 3
Financial assets:
Commercial real estate debt and preferred equity	1,518,165		1,583,969
Financial liabilities:
Participation sold	13,693		14,065
Carrying (Reported) Amount, Fair Value Disclosure | Level 1, Level 2
Financial assets:
Other derivatives	5,499		146,725
Financial liabilities:
Repurchase agreements	71,361,926		61,781,001
Other derivatives	$ 8,027		$ 55,518