Derivative Instruments (Derivative Call Options, Notional Amount and Fair Value, by Counterparty) (Details) - Call Options - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional amount	$ 39,391,037	$ 40,047,196
Fair value	672,958	1,355,989
Bank of America
Derivative [Line Items]
Notional amount	2,038,746	2,680,543
Fair value	36,929	80,692
Barclays
Derivative [Line Items]
Notional amount	6,458,090	5,753,868
Fair value	191,581	217,536
Canadian Imperial Bank of Commerce
Derivative [Line Items]
Notional amount	4,080,916	4,110,525
Fair value	110,616	154,917
Citibank, N.A.
Derivative [Line Items]
Notional amount	3,917,488	4,075,544
Fair value	41,372	109,046
Credit Suisse
Derivative [Line Items]
Notional amount	4,442,148	4,526,414
Fair value	21,101	116,659
J.P. Morgan
Derivative [Line Items]
Notional amount	4,962,629	4,703,234
Fair value	20,842	151,651
Morgan Stanley
Derivative [Line Items]
Notional amount	2,535,021	1,886,995
Fair value	25,478	41,253
Royal Bank of Canada
Derivative [Line Items]
Notional amount	1,776,753	2,565,202
Fair value	35,503	101,511
Societe Generale
Derivative [Line Items]
Notional amount	1,731,241	3,280,286
Fair value	21,407	139,101
SunTrust
Derivative [Line Items]
Notional amount	2,175,542	2,051,229
Fair value	53,077	74,910
Wells Fargo
Derivative [Line Items]
Notional amount	5,106,215	4,221,408
Fair value	113,688	163,520
Exchange Traded
Derivative [Line Items]
Notional amount	166,248	191,948
Fair value	$ 1,364	$ 5,193