Summary of Portfolio of Securities Available for Sale (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 3,464,279	[1]	$ 105,567
Gross Unrealized Gain		81,594	[1]	1,980
Gross Unrealized Loss		14,446	[1]	3,266
Fair Value		3,531,427	[1]	104,281
Mortgage-Related Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,559,961
Gross Unrealized Gain		60,810
Gross Unrealized Loss		2,025
Fair Value		2,618,746
Mortgage-Related Securities | GSE certificates
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[2]	2,023,677		7,786
Gross Unrealized Gain	[2]	46,364
Gross Unrealized Loss	[2]	1,199		460
Fair Value	[2]	2,068,842		7,326
Mortgage-Related Securities | GSE CMOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[3]	536,284
Gross Unrealized Gain	[3]	14,446
Gross Unrealized Loss	[3]	826
Fair Value	[3]	549,904
Other Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		904,318		97,781
Gross Unrealized Gain		20,784		1,980
Gross Unrealized Loss		12,421		2,806
Fair Value		912,681		96,955
Other Securities | U.S. Treasury obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		199,960
Gross Unrealized Loss		62
Fair Value		199,898
Other Securities | GSE debentures
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		473,879
Gross Unrealized Gain		2,044
Gross Unrealized Loss		2,665
Fair Value		473,258
Other Securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		79,702
Gross Unrealized Gain		11,073
Fair Value		90,775
Other Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		70,381		583
Gross Unrealized Gain		540		48
Gross Unrealized Loss		801
Fair Value		70,120		631
Other Securities | Capital trust notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		48,230		9,458
Gross Unrealized Gain		6,498		2
Gross Unrealized Loss		8,632		2,217
Fair Value		46,096		7,243
Other Securities | Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		15,292		70,866
Gross Unrealized Gain		142		1,446
Gross Unrealized Loss				328
Fair Value		15,434		71,984
Other Securities | Mutual Funds and Common Stock
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[4]	16,874		16,874
Gross Unrealized Gain	[4]	487		484
Gross Unrealized Loss	[4]	261		261
Fair Value	[4]	$ 17,100		$ 17,097

[1] The amortized cost includes the non-credit portion of OTTI recorded in AOCL. At December 31, 2017, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).

[2] Government-sponsored enterprise.

[3] Collateralized mortgage obligations.

[4] Primarily consists of mutual funds that are Community Reinvestment Act-qualified investments.