Investments (Narrative) (Details)			12 Months Ended
	Nov. 01, 2020 USD ($)	Nov. 01, 2019 USD ($)	Dec. 31, 2020 USD ($) investment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Investments [Line Items]
Investment securities available for sale transferred to held to maturity fair value, net of tax	$ 5,900,000		$ 5,894,000	$ 72,512,000	$ (109,507,000)
Reclassification of unrealized losses, net of tax	4,500,000		4,157,000	71,149,000	$ (71,149,000)
Interest receivable, available-for-sale debt securities			17,600,000
Interest receivable, held-to-maturity debt securities			5,400,000
Debt securities, accrued interest, writeOff			$ 0
Number of investments in continuous unrealized loss position for more than twelve months | investment			39
Unrealized losses related to marketability of securities or issuers ability to honor redemption obligations			$ 0
Investment value deemed to be OTTI			0
Investment securities, aggregate carrying value, pledged as collateral			4,640,000,000	$ 3,930,000,000
Debt securities, held-to-maturity, nonaccrual			0
Debt securities, held-to-maturity, past due			$ 0
Mortgage Backed Securities
Schedule of Investments [Line Items]
Investment securities available for sale transferred to held to maturity amortized cost	1,460,000,000
Investment securities available for sale transferred to held to maturity fair value	$ 1,470,000,000
Weighted average contractual maturity (years)	18 years
Accounting Standards Update 2019-04 | Mortgage Backed Securities
Schedule of Investments [Line Items]
Investment securities available for sale transferred to held to maturity fair value		$ 2,150,000,000
Mortgage backed securities, held-to-maturity, transferred to available-for-sale		2,080,000,000.00
Investment securities available for sale transferred to held to maturity fair value, net of tax		72,500,000
Reclassification of unrealized losses, net of tax		$ 55,800,000