Investment Securities (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on impairment watch list	$ 47,200	$ 51,600
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,400,000	$ 1,700,000
AFS securities in loss position at period-end, change in unrealized loss	18,500
Securities pledged as collateral	3,900,000
Securities pledged as collateral and may be sold or re-pledged	$ 209,300
Number of investments in a single issuer that exceed 10% of stockholder's equity	0
No investment in a single issuer exceeds this percentage of stockholder's equity	10.00%
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 31,400
U.S. Government and Federal Agency Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
U.S. Treasury inflation-protected securities held, at fair value	433,600
State and Municipal Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Auction rate securities held at fair value	8,400
Private Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value adjustments, net	(13,000)
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities, FV-NI, Unrealized Loss	$ 295