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Securities (Tables) | 12 Months Ended
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Dec. 31, 2019
Marketable Securities [Abstract] |
Trading Securities [Table Text Block] | The fair value and net unrealized gain (loss) included in trading securities is as follows (in thousands):
|  | December 31, 2019 |  | December 31, 2018
|  | Fair Value |  | Net Unrealized Gain (Loss) |  | Fair Value |  | Net Unrealized Gain (Loss)
U.S. government agency debentures |  | $ | 44,264 |  |  | $ | 6 |  |  | $ | 63,765 |  |  | $ | 254 |
Residential agency mortgage-backed securities |  | 1,504,651 |  |  | 2,293 |  |  | 1,791,584 |  |  | 9,966 |
Municipal and other tax-exempt securities |  | 26,196 |  |  | 60 |  |  | 34,507 |  |  | (1 | )
Asset-backed securities |  | 14,084 |  |  | (21 | ) |  | 42,656 |  |  | 685 |
Other debt securities |  | 34,726 |  |  | 21 |  |  | 24,411 |  |  | 65 |
Total trading securities |  | $ | 1,623,921 |  |  | $ | 2,359 |  |  | $ | 1,956,923 |  |  | $ | 10,969 |
Investment Securities (Held-to-Maturity) [Table Text Block] | The amortized cost and fair values of investment securities are as follows (in thousands):
|  | December 31, 2019
|  | Amortized |  | Fair |  | Gross Unrealized
|  | Cost |  | Value |  | Gain |  | Loss
Municipal and other tax-exempt securities |  | $ | 93,653 |  |  | $ | 96,897 |  |  | $ | 3,250 |  |  | $ | (6 | )
Residential agency mortgage-backed securities |  | 10,676 |  |  | 11,164 |  |  | 488 |  |  | — |
Other debt securities |  | 189,089 |  |  | 206,341 |  |  | 17,547 |  |  | (295 | )
Total investment securities |  | $ | 293,418 |  |  | $ | 314,402 |  |  | $ | 21,285 |  |  | $ | (301 | )
|  | December 31, 2018
|  | Amortized |  | Fair |  | Gross Unrealized
|  | Cost |  | Value |  | Gain |  | Loss
Municipal and other tax-exempt securities |  | $ | 137,296 |  |  | $ | 138,562 |  |  | $ | 1,858 |  |  | $ | (592 | )
Residential agency mortgage-backed securities |  | 12,612 |  |  | 12,770 |  |  | 293 |  |  | (135 | )
Other debt securities |  | 205,279 |  |  | 215,966 |  |  | 12,257 |  |  | (1,570 | )
Total investment securities |  | $ | 355,187 |  |  | $ | 367,298 |  |  | $ | 14,408 |  |  | $ | (2,297 | )
The amortized cost and fair values of investment securities at December 31, 2019, by contractual maturity, are as shown in the following table (dollars in thousands):
|  | Less than One Year |  | One to Five Years |  | Six to Ten Years |  | Over Ten Years |  | Total |  | Weighted Average Maturity 1
Fixed maturity debt securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost |  | $ | 41,670 |  |  | $ | 84,027 |  |  | $ | 144,705 |  |  | $ | 12,340 |  |  | $ | 282,742 |  |  | 5.10 |
Fair value |  | 41,840 |  |  | 87,883 |  |  | 161,178 |  |  | 12,337 |  |  | 303,238 |  |  |  |
Residential mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost |  |  |  |  |  |  |  |  |  |  |  |  |  | $ | 10,676 |  |  | 2
Fair value |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,164 |  |  |  |
Total investment securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost |  |  |  |  |  |  |  |  |  |  |  |  |  | $ | 293,418 |  |  |  |
Fair value |  |  |  |  |  |  |  |  |  |  |  |  |  | 314,402 |  |  |  |
1 | Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without penalty.
2 | The average expected lives of residential mortgage-backed securities were 5.0 years based upon current prepayment assumptions.
Temporarily Impaired Investment Securities (in thousands):
|  | December 31, 2019
|  | Number of Securities |  | Less Than 12 Months |  | 12 Months or Longer |  | Total
|  |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss
Investment: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal and other tax-exempt |  | 4 |  |  | $ | 1,001 |  |  | $ | 1 |  |  | $ | 1,706 |  |  | $ | 5 |  |  | $ | 2,707 |  |  | $ | 6 |
Residential agency mortgage-backed securities |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Other debt securities |  | 13 |  |  | 275 |  |  | 1 |  |  | 8,041 |  |  | 294 |  |  | 8,316 |  |  | 295 |
Total investment securities |  | 17 |  |  | $ | 1,276 |  |  | $ | 2 |  |  | $ | 9,747 |  |  | $ | 299 |  |  | $ | 11,023 |  |  | $ | 301 |
|  | December 31, 2018
|  | Number of Securities |  | Less Than 12 Months |  | 12 Months or Longer |  | Total
|  |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss
Investment: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal and other tax- exempt |  | 72 |  |  | $ | 18,255 |  |  | $ | 69 |  |  | $ | 66,141 |  |  | $ | 523 |  |  | $ | 84,396 |  |  | $ | 592 |
Residential agency mortgage-backed securities |  | 2 |  |  | — |  |  | — |  |  | 5,633 |  |  | 135 |  |  | 5,633 |  |  | 135 |
Other debt securities |  | 72 |  |  | 13,372 |  |  | 64 |  |  | 23,028 |  |  | 1,506 |  |  | 36,400 |  |  | 1,570 |
Total investment securities |  | 146 |  |  | $ | 31,627 |  |  | $ | 133 |  |  | $ | 94,802 |  |  | $ | 2,164 |  |  | $ | 126,429 |  |  | $ | 2,297 |
Available for Sale Securities [Table Text Block] | The amortized cost and fair value of available for sale securities are as follows (in thousands):
|  | December 31, 2019
|  | Amortized |  | Fair |  | Gross Unrealized
|  | Cost |  | Value |  | Gain |  | Loss
U.S. Treasury |  | $ | 1,598 |  |  | $ | 1,600 |  |  | $ | 2 |  |  | $ | — |
Municipal and other tax-exempt |  | 1,789 |  |  | 1,861 |  |  | 72 |  |  | — |
Mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |
Residential agency |  | 7,956,297 |  |  | 8,046,096 |  |  | 104,912 |  |  | (15,113 | )
Residential non-agency |  | 25,968 |  |  | 41,609 |  |  | 15,641 |  |  | — |
Commercial agency |  | 3,145,342 |  |  | 3,178,005 |  |  | 37,808 |  |  | (5,145 | )
Other debt securities |  | 500 |  |  | 472 |  |  | — |  |  | (28 | )
Total available for sale securities |  | $ | 11,131,494 |  |  | $ | 11,269,643 |  |  | $ | 158,435 |  |  | $ | (20,286 | )
|  | December 31, 2018
|  | Amortized |  | Fair |  | Gross Unrealized
|  | Cost |  | Value |  | Gain |  | Loss
U.S. Treasury |  | $ | 496 |  |  | $ | 493 |  |  | $ | — |  |  | $ | (3 | )
Municipal and other tax-exempt |  | 2,782 |  |  | 2,864 |  |  | 82 |  |  | — |
Mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |
Residential agency |  | 5,886,323 |  |  | 5,804,708 |  |  | 16,149 |  |  | (97,764 | )
Residential non-agency |  | 40,948 |  |  | 59,736 |  |  | 18,788 |  |  | — |
Commercial agency |  | 2,986,297 |  |  | 2,953,889 |  |  | 7,955 |  |  | (40,363 | )
Other debt securities |  | 35,545 |  |  | 35,430 |  |  | 12 |  |  | (127 | )
Total available for sale securities |  | $ | 8,952,391 |  |  | $ | 8,857,120 |  |  | $ | 42,986 |  |  | $ | (138,257 | )
The amortized cost and fair values of available for sale securities at December 31, 2019, by contractual maturity, are as shown in the following table (dollars in thousands):
| Less than One Year |  | One to Five Years |  | Six to Ten Years |  | Over Ten Years |  | Total |  | Weighted Average Maturity 1
Fixed maturity debt securities: |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost | $ | 5,105 |  |  | $ | 1,113,463 |  |  | $ | 1,455,340 |  |  | $ | 575,321 |  |  | $ | 3,149,229 |  |  | 8.15 |
Fair value | 5,114 |  |  | 1,122,392 |  |  | 1,474,002 |  |  | 580,430 |  |  | 3,181,938 |  |  |
Residential mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost |  |  |  |  |  |  |  |  |  |  |  |  | $ | 7,982,265 |  |  | 2
Fair value |  |  |  |  |  |  |  |  |  |  |  |  | 8,087,705 |  |  |
Total available-for-sale securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortized cost |  |  |  |  |  |  |  |  |  |  |  |  | $ | 11,131,494 |  |  |  |
Fair value |  |  |  |  |  |  |  |  |  |  |  |  | 11,269,643 |  |  |  |
1 | Expected maturities may differ from contractual maturities, because borrowers may have the right to call or prepay obligations with or without penalty.
2 | The average expected lives of mortgage-backed securities were 4.0 years based upon current prepayment assumptions.
Sales of available for sale securities resulted in gains and losses as follows (in thousands):
| Year Ended December 31,
| 2019 |  | 2018 |  | 2017
Proceeds | $ | 1,211,718 |  |  | $ | 745,643 |  |  | $ | 1,309,215 |
Gross realized gains | 14,996 |  |  | 7,117 |  |  | 10,223 |
Gross realized losses | (9,399 | ) |  | (9,918 | ) |  | (5,795 | )
Related federal and state income tax expense (benefit) | 1,425 |  |  | (713 | ) |  | 1,722 |
Debt Securities, Available for Sale, Unrealized Loss Position [Table Text Block] | Temporarily Impaired Available for Sale Securities (In thousands)
|  | December 31, 2019
|  | Number of Securities |  | Less Than 12 Months |  | 12 Months or Longer |  | Total
|  |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss
Available for sale: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Residential agency |  | 133 |  |  | $ | 1,352,597 |  |  | $ | 6,690 |  |  | $ | 686,002 |  |  | $ | 8,423 |  |  | $ | 2,038,599 |  |  | $ | 15,113 |
Commercial agency |  | 69 |  |  | 830,047 |  |  | 4,238 |  |  | 210,877 |  |  | 907 |  |  | 1,040,924 |  |  | 5,145 |
Other debt securities |  | 1 |  |  | — |  |  | — |  |  | 472 |  |  | 28 |  |  | 472 |  |  | 28 |
Total available for sale securities |  | 203 |  |  | $ | 2,182,644 |  |  | $ | 10,928 |  |  | $ | 897,351 |  |  | $ | 9,358 |  |  | $ | 3,079,995 |  |  | $ | 20,286 |
|  | December 31, 2018
|  | Number of Securities |  | Less Than 12 Months |  | 12 Months or Longer |  | Total
|  |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss |  | Fair Value |  | Unrealized Loss
Available for sale: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury |  | 1 |  |  | $ | — |  |  | $ | — |  |  | $ | 493 |  |  | $ | 3 |  |  | $ | 493 |  |  | $ | 3 |
Mortgage-backed securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Residential agency |  | 289 |  |  | 510,824 |  |  | 1,158 |  |  | 3,641,370 |  |  | 96,606 |  |  | 4,152,194 |  |  | 97,764 |
Commercial agency |  | 197 |  |  | 179,258 |  |  | 394 |  |  | 1,969,504 |  |  | 39,969 |  |  | 2,148,762 |  |  | 40,363 |
Other debt securities |  | 3 |  |  | 9,982 |  |  | 63 |  |  | 20,436 |  |  | 64 |  |  | 30,418 |  |  | 127 |
Total available for sale securities |  | 490 |  |  | $ | 700,064 |  |  | $ | 1,615 |  |  | $ | 5,631,803 |  |  | $ | 136,642 |  |  | $ | 6,331,867 |  |  | $ | 138,257 |
Fair Value Option Securities [Table Text Block] | The fair value and net unrealized gain (loss) included in fair value option securities is as follows (in thousands):
|  | December 31, 2019 |  | December 31, 2018
|  | Fair Value |  | Net Unrealized Gain (Loss) |  | Fair Value |  | Net Unrealized Gain (Loss)
U.S. Treasury |  | $ | 9,917 |  |  | $ | (48 | ) |  | $ | — |  |  | $ | — |
Residential agency mortgage-backed securities |  | 1,088,660 |  |  | 14,109 |  |  | 283,235 |  |  | 2,766 |
Total |  | $ | 1,098,577 |  |  | $ | 14,061 |  |  | $ | 283,235 |  |  | $ | 2,766 |

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