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Summary of Portfolio of Securities Available for Sale and Equity Investments with Readily Determinable Fair Values (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | $ 4,167,877 | $ 3,464,279 | [1]
Gross Unrealized Gain |  | 35,116 | 81,594 | [1]
Gross Unrealized Loss |  | 48,344 | 14,446 | [1]
Fair Value |  | 4,154,649 | 3,531,427 | [1]
Mortgage-Related Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 2,595,384 | 2,559,961 |
Gross Unrealized Gain |  | 17,062 | 60,810 |
Gross Unrealized Loss |  | 29,258 | 2,025 |
Fair Value |  | 2,583,188 | 2,618,746 |
Mortgage-Related Securities | GSE certificates |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 1,936,949 | 2,023,677 |
Gross Unrealized Gain |  | 11,307 | 46,364 |
Gross Unrealized Loss |  | 24,840 | 1,199 |
Fair Value |  | 1,923,416 | 2,068,842 |
Mortgage-Related Securities | GSE CMOs |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 658,435 | 536,284 |
Gross Unrealized Gain |  | 5,755 | 14,446 |
Gross Unrealized Loss |  | 4,418 | 826 |
Fair Value |  | 659,772 | 549,904 |
Other Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 904,318 |
Gross Unrealized Gain |  |  | 20,784 |
Gross Unrealized Loss |  |  | 12,421 |
Fair Value |  |  | 912,681 |
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 | Municipal bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 70,381 |
Gross Unrealized Gain |  |  | 540 |
Gross Unrealized Loss |  |  | 801 |
Fair Value |  |  | 70,120 |
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 | Capital trust notes |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 48,230 |
Gross Unrealized Gain |  |  | 6,498 |
Gross Unrealized Loss |  |  | 8,632 |
Fair Value |  |  | 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 |
Debt Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 1,540,327 |  |
Gross Unrealized Gain |  | 17,708 |  |
Gross Unrealized Loss |  | 18,340 |  |
Fair Value |  | 1,539,695 |  |
Debt Securities | U.S. Treasury obligations |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 199,839 |  |
Gross Unrealized Loss |  | 143 |  |
Fair Value |  | 199,696 |  |
Debt Securities | GSE debentures |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 562,866 |  |
Gross Unrealized Gain |  | 929 |  |
Gross Unrealized Loss |  | 9,146 |  |
Fair Value |  | 554,649 |  |
Debt Securities | Municipal bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 69,546 |  |
Gross Unrealized Gain |  | 178 |  |
Gross Unrealized Loss |  | 2,119 |  |
Fair Value |  | 67,605 |  |
Debt Securities | Corporate bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 379,069 |  |
Gross Unrealized Gain |  | 9,922 |  |
Gross Unrealized Loss |  | 898 |  |
Fair Value |  | 388,093 |  |
Debt Securities | Capital trust notes |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 48,252 |  |
Gross Unrealized Gain |  | 6,478 |  |
Gross Unrealized Loss |  | 5,876 |  |
Fair Value |  | 48,854 |  |
Debt Securities | Asset backed securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [3] | 280,755 |  |
Gross Unrealized Gain | [3] | 201 |  |
Gross Unrealized Loss | [3] | 158 |  |
Fair Value | [3] | 280,798 |  |
Mortgage Backed Securities And Other Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [4] | 4,135,711 |  |
Gross Unrealized Gain | [4] | 34,770 |  |
Gross Unrealized Loss | [4] | 47,598 |  |
Fair Value | [4] | 4,122,883 |  |
Equity securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 32,166 |  |
Gross Unrealized Gain |  | 346 |  |
Gross Unrealized Loss |  | 746 |  |
Fair Value |  | 31,766 |  |
Equity securities | Preferred stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 15,292 |  |
Gross Unrealized Loss |  | 184 |  |
Fair Value |  | 15,108 |  |
Equity securities | Mutual Funds and Common Stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [5] | 16,874 |  |
Gross Unrealized Gain | [5] | 346 |  |
Gross Unrealized Loss | [5] | 562 |  |
Fair Value | [5] | $ 16,658 |  |
[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] | Asset-backed securities are substantially U.S. Government guaranteed.
[4] | The amortized cost includes the non-credit portion of OTTI recorded in AOCL. At June 30, 2018, the non-credit portion of OTTI recorded in AOCL was $8.6 million before taxes.
[5] | Primarily consists of mutual funds that are CRA-qualified investments.

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