Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 4,208,445	$ 5,070,517
Gross Amounts Offset in the Statements of Financial Position	(4,208,445)	(4,919,486)
Net Amounts Presented in the Statements of Financial Position	0	151,031
Financial Instruments/Collateral	0	0
Net Amount	0	151,031
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	327,000	0
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	327,000	0
Financial Instruments/Collateral	(327,000)	0
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	4,535,445	5,070,517
Gross Amounts Offset in the Statements of Financial Position	(4,208,445)	(4,919,486)
Net Amounts Presented in the Statements of Financial Position	327,000	151,031
Financial Instruments/Collateral	(327,000)	0
Net Amount	0	151,031
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	3,825,280	1,941,403
Gross Amounts Offset in the Statements of Financial Position	(3,818,167)	(1,941,403)
Net Amounts Presented in the Statements of Financial Position	7,113	0
Financial Instruments/Collateral	(3,781)	0
Net Amount	3,332	0
Securities sold under agreement to repurchase
Gross Amounts of Recognized Financial Instruments	0	0
Gross amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	0	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	3,825,280	1,941,403
Gross amounts Offset in the Statements of Financial Position Agreements to Repurchase, Securities Loaned, Asset	(3,818,167)	(1,941,403)
Net Amounts Presented in the Statements of Financial Position	7,113	0
Financial Instruments/Collateral	(3,781)	0
Net Amount	$ 3,332	$ 0