Derivative Instruments (Derivative Call Options, Notional Amount and Fair Value, by Counterparty) (Details) - Call Options [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional amount	$ 30,589,840	$ 29,277,569
Derivative instruments	706,007	337,256
Bank of America [Member]
Derivative [Line Items]
Notional amount	6,367,855	6,257,861
Derivative instruments	156,209	67,662
Barclays [Member]
Derivative [Line Items]
Notional amount	2,485,187	2,463,768
Derivative instruments	62,339	35,273
BNP Paribas [Member]
Derivative [Line Items]
Notional amount	891,487	1,520,710
Derivative instruments	16,050	16,944
Citibank, N.A. [Member]
Derivative [Line Items]
Notional amount	3,738,323	3,786,498
Derivative instruments	69,484	23,587
Credit Suisse [Member]
Derivative [Line Items]
Notional amount	1,652,310	1,278,492
Derivative instruments	35,776	12,508
Deutsche Bank [Member]
Derivative [Line Items]
Notional amount	522,531	1,349,002
Derivative instruments	7,950	10,704
J.P. Morgan [Member]
Derivative [Line Items]
Notional amount	1,622,978	838,982
Derivative instruments	19,165	5,283
Morgan Stanley [Member]
Derivative [Line Items]
Notional amount	2,870,679	3,465,457
Derivative instruments	49,172	33,171
Royal Bank of Canada [Member]
Derivative [Line Items]
Notional amount	3,413,277	2,820,410
Derivative instruments	91,977	48,654
SunTrust [Member]
Derivative [Line Items]
Notional amount	2,297,155	1,308,434
Derivative instruments	63,758	20,028
Wells Fargo [Member]
Derivative [Line Items]
Notional amount	4,521,020	4,187,955
Derivative instruments	129,851	63,442
Exchange Traded [Member]
Derivative [Line Items]
Notional amount	207,038	0
Derivative instruments	$ 4,276	$ 0