Investment Securities (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	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	$ 52,100		$ 51,600
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	554,300		$ 1,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	13,100
AFS securities in loss position at period-end, change in unrealized loss	197
Securities pledged as collateral	4,600,000
Securities pledged as collateral and may be sold or re-pledged	$ 205,600
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%
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	$ 419,900
Private Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value adjustments, net	(20,500)
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Equity Securities, FV-NI, Unrealized Gain (Loss)	(218)	$ 262
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gains realized on sales	$ 3,291	$ 2,249