Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Resale Agreements and Repurchase Agreements) (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Resale agreements
Gross resale agreements	$ 1,210,000,000			$ 1,210,000,000		$ 1,110,000,000
Weighted-average yields	1.72%		2.59%	2.09%	2.71%
Repurchase agreements
Gross repurchase agreements	$ 348,063,000			$ 348,063,000		$ 450,000,000
Weighted-average rates	2.70%		4.68%	3.48%	4.87%
Repurchase agreements’ extinguishment costs	$ 0	$ 8,700,000	$ 0	$ 8,740,000	$ 0
Extinguishment of repurchase agreements		$ 150,000,000.0
Maturity, 2020	48,100,000			48,100,000
Maturity, 2023	$ 300,000,000.0			$ 300,000,000.0