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Summary of Portfolio of Securities Held to Maturity (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  | $ 521,000 | $ 3,721,368 | [1]
Carrying Amount | [1] |  | 3,712,776 |
Gross Unrealized Gain | [1] |  | 117,373 |
Gross Unrealized Loss | [1] |  | 16,190 |
Fair Value | [1] |  | 3,813,959 |
Mortgage-Related Securities |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 3,212,563 |
Carrying Amount |  |  | 3,212,563 |
Gross Unrealized Gain |  |  | 101,326 |
Gross Unrealized Loss |  |  | 2,456 |
Fair Value |  |  | 3,311,433 |
Mortgage-Related Securities | GSE certificates |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 2,193,489 |
Carrying Amount |  |  | 2,193,489 |
Gross Unrealized Gain |  |  | 64,431 |
Gross Unrealized Loss |  |  | 2,399 |
Fair Value |  |  | 2,255,521 |
Mortgage-Related Securities | GSE CMOs |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost | [2] |  | 1,019,074 |
Carrying Amount | [2] |  | 1,019,074 |
Gross Unrealized Gain | [2] |  | 36,895 |
Gross Unrealized Loss | [2] |  | 57 |
Fair Value | [2] |  | 1,055,912 |
Other Securities |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 508,805 |
Carrying Amount |  |  | 500,213 |
Gross Unrealized Gain |  |  | 16,047 |
Gross Unrealized Loss |  |  | 13,734 |
Fair Value |  |  | 502,526 |
Other Securities | U.S. Treasury obligations |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 200,293 |
Carrying Amount |  |  | 200,293 |
Gross Unrealized Loss |  |  | 73 |
Fair Value |  |  | 200,220 |
Other Securities | GSE debentures |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 88,457 |
Carrying Amount |  |  | 88,457 |
Gross Unrealized Gain |  |  | 3,836 |
Fair Value |  |  | 92,293 |
Other Securities | Corporate bonds |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 74,217 |
Carrying Amount |  |  | 74,217 |
Gross Unrealized Gain |  |  | 9,549 |
Fair Value |  |  | 83,766 |
Other Securities | Municipal bonds |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 71,554 |
Carrying Amount |  |  | 71,554 |
Gross Unrealized Loss |  |  | 1,789 |
Fair Value |  |  | 69,765 |
Other Securities | Capital trust notes |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |
Amortized Cost |  |  | 74,284 |
Carrying Amount |  |  | 65,692 |
Gross Unrealized Gain |  |  | 2,662 |
Gross Unrealized Loss |  |  | 11,872 |
Fair Value |  |  | $ 56,482 |
[1] | Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).
[2] | Collateralized mortgage obligations.

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