Derivative Instruments (Derivative Call Options, Notional Amount and Fair Value, by Counterparty) (Details) - Call Options - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional amount	$ 40,016,230	$ 40,047,196
Fair value	207,265	1,355,989
Bank of America
Derivative [Line Items]
Notional amount	2,025,121	2,680,543
Fair value	5,291	80,692
Barclays
Derivative [Line Items]
Notional amount	6,074,493	5,753,868
Fair value	61,394	217,536
Canadian Imperial Bank of Commerce
Derivative [Line Items]
Notional amount	3,945,969	4,110,525
Fair value	37,967	154,917
Citibank, N.A.
Derivative [Line Items]
Notional amount	3,639,640	4,075,544
Fair value	2,823	109,046
Credit Suisse
Derivative [Line Items]
Notional amount	5,042,247	4,526,414
Fair value	4,697	116,659
J.P. Morgan
Derivative [Line Items]
Notional amount	5,012,755	4,703,234
Fair value	7,464	151,651
Morgan Stanley
Derivative [Line Items]
Notional amount	1,931,969	1,886,995
Fair value	3,180	41,253
Royal Bank of Canada
Derivative [Line Items]
Notional amount	2,197,796	2,565,202
Fair value	18,753	101,511
Societe Generale
Derivative [Line Items]
Notional amount	2,795,508	3,280,286
Fair value	13,928	139,101
SunTrust
Derivative [Line Items]
Notional amount	2,212,338	2,051,229
Fair value	16,438	74,910
Wells Fargo
Derivative [Line Items]
Notional amount	4,942,226	4,221,408
Fair value	34,628	163,520
Exchange Traded
Derivative [Line Items]
Notional amount	196,168	191,948
Fair value	$ 702	$ 5,193