Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Balance Sheet Offsetting) (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Assets, Resale Agreements
Gross Amounts of Recognized Assets	$ 1,210,000,000	$ 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	1,210,000,000	860,000,000
Securities Purchased under Agreements to Resell Gross Amounts Not Offset [Abstract]
Collateral Received	(1,204,933,000)	(856,058,000)
Net Amount	5,067,000	3,942,000
Liabilities, Repurchase Agreements
Gross Amounts of Recognized Liabilities	348,063,000	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	348,063,000	200,000,000
Securities Sold under Agreements to Repurchase Gross Amounts Not Offset [Abstract]
Collateral Pledged	(330,600,000)	(200,000,000)
Net Amount	$ 17,463,000	$ 0