Assets Purchased under Resale Agreements and Sold under Repurchase Agreements (Balance Sheet Offsetting) (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets, Resale Agreements
Gross Amounts of Recognized Assets	$ 1,460,000,000	$ 1,110,000,000
Gross Amounts Offset on the Consolidated Balance Sheet	0	(250,000,000)
Net Amounts of Assets Presented on the Consolidated Balance Sheet	1,460,000,000	860,000,000
Gross Amounts Not Offset on the Consolidated Balance Sheet
Collateral Received	(1,458,700,000)	(856,058,000)
Net Amount	1,300,000	3,942,000
Liabilities, Repurchase Agreements
Gross Amounts of Recognized Liabilities	300,000,000	450,000,000
Gross Amounts Offset on the Consolidated Balance Sheet	0	(250,000,000)
Net Amounts of Liabilities Presented on the Consolidated Balance Sheet	300,000,000	200,000,000
Gross Amounts Not Offset on the Consolidated Balance Sheet
Collateral Pledged	(300,000,000)	(200,000,000)
Net Amount	$ 0	$ 0