Derivative Instruments (Derivative Call Options, Notional Amount and Fair Value, by Counterparty) (Details) - Call Options - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional amount	$ 38,938,332	$ 40,047,196
Fair value	874,741	1,355,989
Bank of America
Derivative [Line Items]
Notional amount	2,534,503	2,680,543
Fair value	59,082	80,692
Barclays
Derivative [Line Items]
Notional amount	6,222,261	5,753,868
Fair value	234,710	217,536
Canadian Imperial Bank of Commerce
Derivative [Line Items]
Notional amount	5,076,462	4,110,525
Fair value	169,396	154,917
Citibank, N.A.
Derivative [Line Items]
Notional amount	3,683,100	4,075,544
Fair value	50,058	109,046
Credit Suisse
Derivative [Line Items]
Notional amount	3,475,584	4,526,414
Fair value	31,566	116,659
J.P. Morgan
Derivative [Line Items]
Notional amount	4,016,877	4,703,234
Fair value	25,190	151,651
Morgan Stanley
Derivative [Line Items]
Notional amount	2,871,578	1,886,995
Fair value	26,384	41,253
Royal Bank of Canada
Derivative [Line Items]
Notional amount	1,440,582	2,565,202
Fair value	31,525	101,511
Societe Generale
Derivative [Line Items]
Notional amount	1,612,361	3,280,286
Fair value	14,632	139,101
Truist
Derivative [Line Items]
Notional amount	2,579,446	2,051,229
Fair value	77,930	74,910
Wells Fargo
Derivative [Line Items]
Notional amount	5,251,699	4,221,408
Fair value	151,799	163,520
Exchange Traded
Derivative [Line Items]
Notional amount	173,879	191,948
Fair value	$ 2,469	$ 5,193