Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 4,979,330	$ 4,872,392
Gross Amounts Offset in the Statements of Financial Position	(4,970,040)	(4,582,540)
Net Amounts Presented in the Statements of Financial Position	9,290	289,852
Financial Instruments/Collateral	0	0
Net Amount	9,290	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	4,979,330	5,127,392
Gross Amounts Offset in the Statements of Financial Position	(4,970,040)	(4,582,540)
Net Amounts Presented in the Statements of Financial Position	9,290	544,852
Financial Instruments/Collateral	0	(255,000)
Net Amount	9,290	289,852
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	1,660,701	1,654,360
Gross Amounts Offset in the Statements of Financial Position	(1,660,701)	(1,654,360)
Net Amounts Presented in the Statements of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0