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Summary of Portfolio of Securities Available for Sale and Equity Investments with Readily Determinable Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | $ 4,838,804 | $ 3,464,279 | [1]
Gross Unrealized Gain |  | 28,110 | 81,594 | [1]
Gross Unrealized Loss |  | 70,907 | 14,446 | [1]
Fair Value |  | 4,796,007 | 3,531,427 | [1]
Mortgage-Related Securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 2,811,486 | 2,559,961 |
Gross Unrealized Gain |  | 8,904 | 60,810 |
Gross Unrealized Loss |  | 39,494 | 2,025 |
Fair Value |  | 2,780,896 | 2,618,746 |
Mortgage-Related Securities | GSE certificates |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 1,892,104 | 2,023,677 |
Gross Unrealized Gain |  | 5,113 | 46,364 |
Gross Unrealized Loss |  | 33,361 | 1,199 |
Fair Value |  | 1,863,856 | 2,068,842 |
Mortgage-Related Securities | GSE CMOs |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 919,382 | 536,284 |
Gross Unrealized Gain |  | 3,791 | 14,446 |
Gross Unrealized Loss |  | 6,133 | 826 |
Fair Value |  | 917,040 | 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 |  | 1,995,152 |  |
Gross Unrealized Gain |  | 18,830 |  |
Gross Unrealized Loss |  | 30,595 |  |
Fair Value |  | 1,983,387 |  |
Debt Securities | GSE debentures |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 1,043,927 |  |
Gross Unrealized Gain |  | 632 |  |
Gross Unrealized Loss |  | 22,868 |  |
Fair Value |  | 1,021,691 |  |
Debt Securities | Municipal bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 69,276 |  |
Gross Unrealized Gain |  | 100 |  |
Gross Unrealized Loss |  | 1,877 |  |
Fair Value |  | 67,499 |  |
Debt Securities | Corporate bonds |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 445,902 |  |
Gross Unrealized Gain |  | 10,384 |  |
Gross Unrealized Loss |  | 655 |  |
Fair Value |  | 455,631 |  |
Debt Securities | Capital trust notes |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 48,265 |  |
Gross Unrealized Gain |  | 6,639 |  |
Gross Unrealized Loss |  | 4,661 |  |
Fair Value |  | 50,243 |  |
Debt Securities | Asset backed securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [3] | 387,782 |  |
Gross Unrealized Gain | [3] | 1,075 |  |
Gross Unrealized Loss | [3] | 534 |  |
Fair Value | [3] | 388,323 |  |
Mortgage Backed Securities And Other Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [4] | 4,806,638 |  |
Gross Unrealized Gain | [4] | 27,734 |  |
Gross Unrealized Loss | [4] | 70,089 |  |
Fair Value | [4] | 4,764,283 |  |
Equity securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 32,166 |  |
Gross Unrealized Gain |  | 376 |  |
Gross Unrealized Loss |  | 818 |  |
Fair Value |  | 31,724 |  |
Equity securities | Preferred stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost |  | 15,292 |  |
Gross Unrealized Loss |  | 138 |  |
Fair Value |  | 15,154 |  |
Equity securities | Mutual Funds and Common Stock |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Amortized Cost | [2] | 16,874 |  |
Gross Unrealized Gain | [2] | 376 |  |
Gross Unrealized Loss | [2] | 680 |  |
Fair Value | [2] | $ 16,570 |  |
[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 underlying assets of the asset-backed securities are substantially guaranteed by the U.S. Government.
[4] | The amortized cost includes the non-credit portion of OTTI recorded in AOCL. At September 30, 2018, the non-credit portion of OTTI recorded in AOCL was $8.6 million before taxes.

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