Derivative Instruments (Schedule of Call Options by Counterparty) (Details) - Call Options [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional amount	$ 29,277,569	$ 23,956,444
Fair value	337,256	731,113
Bank of America [Member]
Derivative [Line Items]
Notional amount	6,257,861	2,114,812
Fair value	67,662	62,932
Barclays [Member]
Derivative [Line Items]
Notional amount	2,463,768	4,083,259
Fair value	35,273	135,609
BNP Paribas [Member]
Derivative [Line Items]
Notional amount	1,520,710	1,321,136
Fair value	16,944	42,644
Citibank, N.A. [Member]
Derivative [Line Items]
Notional amount	3,786,498	3,190,204
Fair value	23,587	96,759
Credit Suisse [Member]
Derivative [Line Items]
Notional amount	1,278,492	2,354,811
Fair value	12,508	75,381
Deutsche Bank [Member]
Derivative [Line Items]
Notional amount	1,349,002	2,682,960
Fair value	10,704	64,028
HSBC [Member]
Derivative [Line Items]
Notional amount	0	38,599
Fair value	0	1,767
J.P. Morgan [Member]
Derivative [Line Items]
Notional amount	838,982	401,804
Fair value	5,283	13,488
Morgan Stanley [Member]
Derivative [Line Items]
Notional amount	3,465,457	2,605,687
Fair value	33,171	77,106
Royal Bank of Canada [Member]
Derivative [Line Items]
Notional amount	2,820,410	1,364,362
Fair value	48,654	41,717
SunTrust [Member]
Derivative [Line Items]
Notional amount	1,308,434	248,622
Fair value	20,028	5,405
Wells Fargo [Member]
Derivative [Line Items]
Notional amount	4,187,955	3,550,188
Fair value	$ 63,442	$ 114,277