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Summary of Portfolio of Securities Available for Sale and Equity Investments with Readily Determinable Fair Values (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [1] |  | $ 3,464,279 |
Gross Unrealized Gain | [1] |  | 81,594 |
Gross Unrealized Loss | [1] |  | 14,446 |
Fair Value |  | $ 3,391,952 | 3,531,427 | [1]
Amortized Cost |  | 3,412,716 |  |
Gross Unrealized Gain |  | 46,181 |  |
Gross Unrealized Loss |  | 34,876 |  |
Fair Value |  | 3,424,021 | 3,531,427 |
Mortgage-Related Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 2,504,097 | 2,559,961 |
Gross Unrealized Gain |  | 27,990 | 60,810 |
Gross Unrealized Loss |  | 19,323 | 2,025 |
Fair Value |  | 2,512,764 | 2,618,746 |
Mortgage-Related Securities | GSE certificates |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 1,966,476 | 2,023,677 |
Gross Unrealized Gain |  | 20,565 | 46,364 |
Gross Unrealized Loss |  | 15,864 | 1,199 |
Fair Value |  | 1,971,177 | 2,068,842 |
Mortgage-Related Securities | GSE CMOs |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 537,621 | 536,284 |
Gross Unrealized Gain |  | 7,425 | 14,446 |
Gross Unrealized Loss |  | 3,459 | 826 |
Fair Value |  | 541,587 | 549,904 |
Other Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 876,453 | 904,318 |
Gross Unrealized Gain |  | 17,789 | 20,784 |
Gross Unrealized Loss |  | 15,054 | 12,421 |
Fair Value |  | 879,188 | 912,681 |
Other Securities | U.S. Treasury obligations |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 199,678 | 199,960 |
Gross Unrealized Loss |  | 248 | 62 |
Fair Value |  | 199,430 | 199,898 |
Other Securities | GSE debentures |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 478,588 | 473,879 |
Gross Unrealized Gain |  | 1,213 | 2,044 |
Gross Unrealized Loss |  | 6,732 | 2,665 |
Fair Value |  | 473,069 | 473,258 |
Other Securities | Corporate bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 79,828 | 79,702 |
Gross Unrealized Gain |  | 9,890 | 11,073 |
Fair Value |  | 89,718 | 90,775 |
Other Securities | Municipal bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 70,117 | 70,381 |
Gross Unrealized Gain |  | 201 | 540 |
Gross Unrealized Loss |  | 1,711 | 801 |
Fair Value |  | 68,607 | 70,120 |
Other Securities | Capital trust notes |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 48,242 | 48,230 |
Gross Unrealized Gain |  | 6,485 | 6,498 |
Gross Unrealized Loss |  | 6,363 | 8,632 |
Fair Value |  | 48,364 | 46,096 |
Other Securities | Preferred stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 15,292 |
Gross Unrealized Gain |  |  | 142 |
Fair Value |  |  | 15,434 |
Other Securities | Mutual Funds and Common Stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [2] |  | 16,874 |
Gross Unrealized Gain | [2] |  | 487 |
Gross Unrealized Loss | [2] |  | 261 |
Fair Value | [2] |  | $ 17,100 |
Mortgage Backed Securities And Other Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [3] | 3,380,550 |  |
Gross Unrealized Gain | [3] | 45,779 |  |
Gross Unrealized Loss | [3] | 34,377 |  |
Fair Value | [3] | 3,391,952 |  |
Equity securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 32,166 |  |
Gross Unrealized Gain |  | 402 |  |
Gross Unrealized Loss |  | 499 |  |
Fair Value |  | 32,069 |  |
Equity securities | Preferred stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 15,292 |  |
Gross Unrealized Loss |  | 49 |  |
Fair Value |  | 15,243 |  |
Equity securities | Mutual Funds and Common Stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [2] | 16,874 |  |
Gross Unrealized Gain | [2] | 402 |  |
Gross Unrealized Loss | [2] | 450 |  |
Fair Value | [2] | $ 16,826 |  |
[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] | Primarily consists of mutual funds that are CRA-qualified investments.
[3] | The amortized cost includes the non-credit portion of OTTI recorded in AOCL. At March 31, 2018, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).

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- Definition This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, which are expected to mature after five years from the balance sheet date and which are categorized as marketable. + References No definition available. + Details
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