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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.

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