Derivative Instruments (Offsetting Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivatives Assets
Gross Amounts of Recognized Financial Instruments	$ 21,808,979	$ 7,324,051
Gross Amounts Offset in the Statements of Financial Position	(21,791,617)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	17,362	2
Financial Instruments/Collateral	0	0
Net Amount	17,362	2
Securities Purchased under Agreements to Resell
Gross Amounts of Recognized Financial Instruments	100,000	302,000
Gross Amounts Offset in the Statements of Financial Position	0	0
Net Amounts Presented in the Statements of Financial Position	100,000	302,000
Financial Instruments/Collateral	(100,000)	(302,000)
Net Amount	0	0
Total Assets
Gross Amounts of Recognized Financial Instruments	21,908,979	7,626,051
Gross Amounts Offset in the Statements of Financial Position	(21,791,617)	(7,324,049)
Net Amounts Presented in the Statements of Financial Position	117,362	302,002
Financial Instruments/Collateral	(100,000)	(302,000)
Net Amount	17,362	2
Derivatives Liabilities
Gross Amounts of Recognized Financial Instruments	8,175,915	3,206,949
Gross Amounts Offset in the Statements of Financial Position	(7,703,646)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	472,269	153,817
Financial Instruments/Collateral	(331,133)	(820)
Net Amount	141,136	152,997
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	8,175,915	3,206,949
Gross amounts Offset in the Statements of Financial Position Agreements to Repurchase, Securities Loaned, Asset	(7,703,646)	(3,053,132)
Net Amounts Presented in the Statements of Financial Position	472,269	153,817
Financial Instruments/Collateral	(331,133)	(820)
Net Amount	$ 141,136	$ 152,997