Derivatives - Interest Rate Swap Contracts, Swaptions and Futures Contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 7,975,000	$ 7,350,000
Weighted Average Remaining Term (Months)	53 months	62 months
Weighted Average Rate	1.74%	1.98%
London Interbank Offered Rate (LIBOR) Swap Rate
Derivative [Line Items]
Notional Amount	$ 1,025,000
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Derivative [Line Items]
Notional Amount	375,000
Fed Funds Effective Rate Overnight Index Swap Rate
Derivative [Line Items]
Notional Amount	6,575,000
3 years
Derivative [Line Items]
Notional Amount	$ 2,750,000	$ 2,225,000
Weighted Average Remaining Term (Months)	19 months	20 months
Weighted Average Rate	1.66%	1.67%
≥ 3 years and 5 years
Derivative [Line Items]
Notional Amount	$ 2,850,000	$ 1,725,000
Weighted Average Remaining Term (Months)	47 months	54 months
Weighted Average Rate	1.84%	1.95%
≥ 5 years and 7 years
Derivative [Line Items]
Notional Amount	$ 1,200,000	$ 925,000
Weighted Average Remaining Term (Months)	83 months	69 months
Weighted Average Rate	1.86%	2.06%
≥ 7 years and 10 years
Derivative [Line Items]
Notional Amount	$ 1,175,000	$ 2,475,000
Weighted Average Remaining Term (Months)	118 months	104 months
Weighted Average Rate	1.54%	2.25%
Minimum | ≥ 3 years and 5 years
Derivative [Line Items]
Remaining / Underlying Term	3 years	3 years
Minimum | ≥ 5 years and 7 years
Derivative [Line Items]
Remaining / Underlying Term	5 years	5 years
Minimum | ≥ 7 years and 10 years
Derivative [Line Items]
Remaining / Underlying Term	7 years	7 years
Maximum | 3 years
Derivative [Line Items]
Remaining / Underlying Term	3 years	3 years
Maximum | ≥ 3 years and 5 years
Derivative [Line Items]
Remaining / Underlying Term	5 years	5 years
Maximum | ≥ 5 years and 7 years
Derivative [Line Items]
Remaining / Underlying Term	7 years	7 years
Maximum | ≥ 7 years and 10 years
Derivative [Line Items]
Remaining / Underlying Term	10 years	10 years
Maximum | ≥ 10 years
Derivative [Line Items]
Remaining / Underlying Term	10 years	10 years