Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 4,965,587	$ 4,872,392
Gross Amounts Offset in the Statements of Financial Position	(4,965,587)	(4,582,540)
Net Amounts Presented in the Statements of Financial Position	0	289,852
Financial Instruments/Collateral	0	0
Net Amount	0	289,852
Securities Purchased under Agreements to Resell [Abstract]
Gross Amounts of Recognized Financial Instruments	0	255,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	0	255,000
Financial Instruments/Collateral	0	(255,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	4,965,587	5,127,392
Gross Amounts Offset in the Statements of Financial Position	(4,965,587)	(4,582,540)
Net Amounts Presented in the Statements of Financial Position	0	544,852
Financial Instruments/Collateral	0	(255,000)
Net Amount	0	289,852
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	1,497,613	1,654,360
Gross Amounts Offset in the Statements of Financial Position	(1,406,616)	(1,654,360)
Net Amounts Presented in the Statements of Financial Position	90,997	0
Financial Instruments/Collateral	(90,997)	0
Net Amount	$ 0	$ 0