Investment Securities (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Investments_In_Single_Issuer	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Investment [Line Items]
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 51,600,000	$ 57,700,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	13,275,000	14,092,000	$ 14,199,000	$ 14,080,000
Portfolio securities aggregate fair value loss position	1,654,114,000	5,435,496,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	12,864,000	99,824,000
Available for sale securities in loss position at period end, change in unrealized loss	(87,000,000.0)
Securities pledged as collateral	4,300,000,000		$ 4,300,000,000
Securities pledged and may be sold or re-pledged	$ 204,900,000
Number of investments in a single issuer that exceed 10% of stockholder's equity | Investments_In_Single_Issuer	0
No investment in a single issuer exceeds this percentage of stockholder's equity	10.00%
Visa Class B Shares [Member]
Investment [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 0
US Treasury and Government [Member]
Investment [Line Items]
U.S. Treasury inflation-protected securities held	461,800,000
Portfolio securities aggregate fair value loss position	57,192,000	434,427,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	42,000	11,379,000
State and Municipal Obligations [Member]
Investment [Line Items]
Auction rate securities held at fair value	9,900,000
Portfolio securities aggregate fair value loss position	8,254,000	414,889,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	32,000	5,030,000
Non-agency mortgage-backed securities [Member]
Investment [Line Items]
Par value securities with other-than-temporary impairment	17,500,000
Fair Value Securities with Other-Than-Temporary Impairment	13,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	13,300,000
Portfolio securities aggregate fair value loss position	292,678,000	865,191,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 705,000	$ 12,779,000