Derivatives - Interest Rate Swap Contracts, Swaptions and Futures Contracts (Details) - USD ($)
|
9 Months Ended |
12 Months Ended |
Sep. 30, 2020 |
Dec. 31, 2019 |
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 4,937,000,000
|
$ 7,975,000,000
|
| Weighted Average Remaining Term (Months) |
55 months
|
53 months
|
| Weighted Average Rate |
0.21%
|
1.74%
|
| Fed Funds Effective Rate Overnight Index Swap Rate |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 2,707,000,000
|
$ 6,575,000,000
|
| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 2,230,000,000
|
375,000,000
|
| London Interbank Offered Rate (LIBOR) Swap Rate |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
|
$ 1,025,000,000
|
| Less than 3 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
3 years
|
3 years
|
| Notional Amount |
$ 2,230,000,000
|
$ 2,750,000,000
|
| Weighted Average Remaining Term (Months) |
15 months
|
19 months
|
| Weighted Average Rate |
0.06%
|
1.66%
|
| Greater than or equal to 3 years and less than 5 years |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 463,000,000
|
$ 2,850,000,000
|
| Weighted Average Remaining Term (Months) |
48 months
|
47 months
|
| Weighted Average Rate |
0.14%
|
1.84%
|
| Greater than or equal to 5 years and less than 7 years |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 942,000,000
|
$ 1,200,000,000
|
| Weighted Average Remaining Term (Months) |
75 months
|
83 months
|
| Weighted Average Rate |
0.28%
|
1.86%
|
| Greater than or equal to 7 years |
|
|
| Derivative [Line Items] |
|
|
| Notional Amount |
$ 1,302,000,000
|
$ 1,175,000,000
|
| Weighted Average Remaining Term (Months) |
113 months
|
118 months
|
| Weighted Average Rate |
0.44%
|
1.54%
|
| Minimum | Greater than or equal to 3 years and less than 5 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
3 years
|
3 years
|
| Minimum | Greater than or equal to 5 years and less than 7 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
5 years
|
5 years
|
| Maximum | Greater than or equal to 3 years and less than 5 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
5 years
|
5 years
|
| Maximum | Greater than or equal to 5 years and less than 7 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
7 years
|
7 years
|
| Maximum | Greater than or equal to 7 years |
|
|
| Derivative [Line Items] |
|
|
| Derivative, remaining maturity |
7 years
|
7 years
|