Derivative Instruments (Schedule of Call Options by Counterparty) (Details) - Call Options - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional amount	$ 40,047,196	$ 39,420,777
Fair value	1,355,989	205,149
Bank of America
Derivative [Line Items]
Notional amount	2,680,543	6,518,808
Fair value	80,692	6,704
Barclays
Derivative [Line Items]
Notional amount	5,753,868	2,301,414
Fair value	217,536	27,032
Canadian Imperial Bank of Commerce
Derivative [Line Items]
Notional amount	4,110,525	4,856,150
Fair value	154,917	29,313
Citibank, N.A.
Derivative [Line Items]
Notional amount	4,075,544	4,792,208
Fair value	109,046	27,239
Credit Suisse
Derivative [Line Items]
Notional amount	4,526,414	2,877,916
Fair value	116,659	12,887
J.P. Morgan
Derivative [Line Items]
Notional amount	4,703,234	3,701,964
Fair value	151,651	17,564
Morgan Stanley
Derivative [Line Items]
Notional amount	1,886,995	3,560,044
Fair value	41,253	1,561
Royal Bank of Canada
Derivative [Line Items]
Notional amount	2,565,202	1,871,305
Fair value	101,511	14,011
Societe Generale
Derivative [Line Items]
Notional amount	3,280,286	2,343,165
Fair value	139,101	21,681
SunTrust
Derivative [Line Items]
Notional amount	2,051,229	1,755,030
Fair value	74,910	12,047
Wells Fargo
Derivative [Line Items]
Notional amount	4,221,408	4,618,569
Fair value	163,520	33,398
Exchange Traded
Derivative [Line Items]
Notional amount	191,948	224,204
Fair value	$ 5,193	$ 1,712