Summary of Portfolio of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016		Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,660,550	[1]	$ 5,978,107	[2]
Carrying Amount	3,651,925	[1]	5,969,390	[2]
Gross Unrealized Gain	255,214	[1]	168,470	[2]
Gross Unrealized Loss	14,053	[1]	29,331	[2]
Fair Value	3,893,086	[1]	6,108,529	[2]
Mortgage-Related Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,351,458		3,594,861
Carrying Amount	3,351,458		3,594,861
Gross Unrealized Gain	231,761		130,063
Gross Unrealized Loss			4,779
Fair Value	3,583,219		3,720,145
Mortgage-Related Securities | GSE certificates
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,231,754		2,269,828
Carrying Amount	2,231,754		2,269,828
Gross Unrealized Gain	156,154		76,827
Gross Unrealized Loss			4,722
Fair Value	2,387,908		2,341,933
Mortgage-Related Securities | GSE CMOs
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,119,704	[3]	1,325,033
Carrying Amount	1,119,704	[3]	1,325,033
Gross Unrealized Gain	75,607	[3]	53,236
Gross Unrealized Loss			57
Fair Value	1,195,311	[3]	1,378,212
Other Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	309,092		2,383,246
Carrying Amount	300,467		2,374,529
Gross Unrealized Gain	23,453		38,407
Gross Unrealized Loss	14,053		24,552
Fair Value	309,867		2,388,384
Other Securities | GSE debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	88,527		2,159,856
Carrying Amount	88,527		2,159,856
Gross Unrealized Gain	6,084		23,892
Gross Unrealized Loss			7,568
Fair Value	94,611		2,176,180
Other Securities | Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	74,101		73,756
Carrying Amount	74,101		73,756
Gross Unrealized Gain	10,753		10,503
Fair Value	84,854		84,259
Other Securities | Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	72,187		75,317
Carrying Amount	72,187		75,317
Gross Unrealized Gain	2,698		262
Gross Unrealized Loss			1,084
Fair Value	74,885		74,495
Other Securities | Capital trust notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	74,277		74,317
Carrying Amount	65,652		65,600
Gross Unrealized Gain	3,918		3,750
Gross Unrealized Loss	14,053		15,900
Fair Value	$ 55,517		$ 53,450

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in Accumulated Other Comprehensive Loss ("AOCL"). At September 30, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million, pre-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, 2015, the non-credit portion of OTTI recorded in AOCL was $8.7 million, pre-tax.

[3] Collateralized mortgage obligations.