Derivative Instruments (Offsetting Balance Sheet (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Derivatives
Gross Amounts of Recognized Financial Instruments	$ 4,896,359,000	$ 136,775,000
Securities purchased under agreement to resell
Gross Amounts of Recognized Financial Instruments	255,000,000
Gross Amounts Offset in the Statements of Financial Position	0
Net Amounts Presented in the Statements of Financial Position	255,000,000
Financial Instruments/Collateral	(255,000,000)
Net Amount	0
Total Assets
Gross Amounts of Recognized Financial Instruments	5,127,392,000
Gross Amounts Offset in the Statements of Financial Position	(4,582,540,000)
Net Amounts Presented in the Statements of Financial Position	544,852,000
Financial Instruments/Collateral	(255,000,000)
Net Amount	289,852,000
Derivatives
Gross Amounts of Recognized Financial Instruments	1,659,411,000	25,277,000
Securities sold under agreement to repurchase
Gross Amounts of Recognized Financial Instruments	0
Gross Amounts Offset in the Statements of Financial Position	0
Net Amounts Presented in the Statements of Financial Position	0
Financial Instruments/Collateral	0
Net Amount	0
Total Liabilities
Gross Amounts of Recognized Financial Instruments	1,654,360,000
Gross Amounts Offset in the Statements of Financial Position	(1,654,360,000)
Net Amounts Presented in the Statements of Financial Position	0
Financial Instruments/Collateral	0
Net Amount	0
Counterparty A Member [Member]
Derivatives
Gross Amounts of Recognized Financial Instruments	4,872,392,000	135,210,000
Gross Amounts Offset in the Statements of Financial Position	(4,582,540,000)	(21,508,000)
Net Amounts Presented in the Statements of Financial Position	289,852,000	113,702,000
Financial Instruments/Collateral	0	(101,288,000)
Net Amount	289,852,000	12,414,000
Derivatives
Gross Amounts of Recognized Financial Instruments	1,654,360,000	25,277,000
Gross Amounts Offset in the Statements of Financial Position	(1,654,360,000)	(25,277,000)
Net Amounts Presented in the Statements of Financial Position	0	0
Financial Instruments/Collateral	0	0
Net Amount	$ 0	$ 0