Summary of Portfolio of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015		Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,606,032	[1]	$ 6,931,498	[2]
Carrying Amount	6,597,285	[1]	6,922,667	[2]
Gross Unrealized Gain	237,066	[1]	212,968	[2]
Gross Unrealized Loss	21,577	[1]	49,664	[2]
Fair Value	6,812,774	[1]	7,085,971	[2]
Mortgage-Related Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,756,336		4,079,034
Carrying Amount	3,756,336		4,079,034
Gross Unrealized Gain	194,842		171,489
Gross Unrealized Loss	761		4,549
Fair Value	3,950,417		4,245,974
Mortgage-Related Securities | GSE certificates
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,351,901		2,468,791
Carrying Amount	2,351,901		2,468,791
Gross Unrealized Gain	118,056		106,414
Gross Unrealized Loss	761		3,838
Fair Value	2,469,196		2,571,367
Mortgage-Related Securities | GSE CMOs
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,404,435		1,610,243
Carrying Amount	1,404,435		1,610,243
Gross Unrealized Gain	76,786		65,075
Gross Unrealized Loss			711
Fair Value	1,481,221		1,674,607
Other Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,849,696		2,852,464
Carrying Amount	2,840,949		2,843,633
Gross Unrealized Gain	42,224		41,479
Gross Unrealized Loss	20,816		45,115
Fair Value	2,862,357		2,839,997
Other Securities | GSE debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,624,600		2,635,989
Carrying Amount	2,624,600		2,635,989
Gross Unrealized Gain	27,247		24,173
Gross Unrealized Loss	4,489		32,920
Fair Value	2,647,358		2,627,242
Other Securities | Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	73,645		73,317
Carrying Amount	73,645		73,317
Gross Unrealized Gain	10,793		12,113
Fair Value	84,438		85,430
Other Securities | Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	77,140		58,682
Carrying Amount	77,140		58,682
Gross Unrealized Gain	145
Gross Unrealized Loss	1,039		1,027
Fair Value	76,246		57,655
Other Securities | Capital trust notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	74,311		84,476
Carrying Amount	65,564		75,645
Gross Unrealized Gain	4,039		5,193
Gross Unrealized Loss	15,288		11,168
Fair Value	$ 54,315		$ 69,670

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At September 30, 2015, the non-credit portion of OTTI recorded in AOCL was $8.7 million (before tax).

[2] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2014, the non-credit portion of OTTI recorded in AOCL was $8.8 million (before taxes).