Available-For-Sale And Held-To-Maturity Securities (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Aug. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Proceeds from sale of available-for-sale securities	$ 486,500	$ 55,400	$ 641,800	$ 55,400
Net realized gains (loss) resulting from sale of available-for-sale securities		(400)	(300)	(400)
Transfers from held to maturity to available for sale securities reclassification fair value					$ 1,100,000
Other comprehensive income (loss), transfers from held-to-maturity to available-for-sale securities, net of tax	17,900		17,900
Unrealized gains (losses) recorded in accumulated other comprehensive loss	(1,600)	(3,100)	40,500	(28,100)
Financial instruments pledged as collateral	$ 675,500		$ 675,500			$ 1,900,000
Number of available for sale securities whose amortized costs exceeded their fair values | security	137		137
Available-for-sale Securities, Continuous	$ 16,137		$ 16,137
Available-for-sale Securities, Continuous Unrealized	5,770		5,770
Available-for-sale Securities, Continuous	$ 1,752,166		$ 1,752,166
Percentage of available-for-sale portfolio	50.80%		50.80%
Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	108		108
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 33,100		$ 33,100
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	29,200		29,200
Credit-related OTTI	0	$ 0	0	$ 0
Gross unrealized losses related to investment portfolio			49,200
Pledged [Member]
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]
Trading securities pledged	$ 946,400		$ 946,400			$ 1,600,000