Summary of Portfolio of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,793,687	[1]	$ 6,931,498	[2]
Carrying Amount	6,784,883	[1]	6,922,667	[2]
Gross Unrealized Gain	265,788	[1]	212,968	[2]
Gross Unrealized Loss	22,470	[1]	49,664	[2]
Fair Value	7,028,201	[1]	7,085,971	[2]
Mortgage-Related Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,955,139		4,079,034
Carrying Amount	3,955,139		4,079,034
Gross Unrealized Gain	218,784		171,489
Gross Unrealized Loss	2,086		4,549
Fair Value	4,171,837		4,245,974
Mortgage-Related Securities | GSE certificates
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,401,201		2,468,791
Carrying Amount	2,401,201		2,468,791
Gross Unrealized Gain	133,896		106,414
Gross Unrealized Loss	2,080		3,838
Fair Value	2,533,017		2,571,367
Mortgage-Related Securities | GSE CMOs
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,553,938		1,610,243
Carrying Amount	1,553,938		1,610,243
Gross Unrealized Gain	84,888		65,075
Gross Unrealized Loss	6		711
Fair Value	1,638,820		1,674,607
Other Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,838,548		2,852,464
Carrying Amount	2,829,744		2,843,633
Gross Unrealized Gain	47,004		41,479
Gross Unrealized Loss	20,384		45,115
Fair Value	2,856,364		2,839,997
Other Securities | GSE debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,632,327		2,635,989
Carrying Amount	2,632,327		2,635,989
Gross Unrealized Gain	29,242		24,173
Gross Unrealized Loss	9,104		32,920
Fair Value	2,652,465		2,627,242
Other Securities | Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	73,426		73,317
Carrying Amount	73,426		73,317
Gross Unrealized Gain	12,446		12,113
Fair Value	85,872		85,430
Other Securities | Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	58,376		58,682
Carrying Amount	58,376		58,682
Gross Unrealized Loss	352		1,027
Fair Value	58,024		57,655
Other Securities | Capital trust notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	74,419		84,476
Carrying Amount	65,615		75,645
Gross Unrealized Gain	5,316		5,193
Gross Unrealized Loss	10,928		11,168
Fair Value	$ 60,003		$ 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 March 31, 2015, the non-credit portion of OTTI recorded in AOCL was $8.8 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).