Investment Securities (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Investment [Line Items]
Equity Securities, FV-NI, Cost	$ 2,500
Equity securities with a readily determinable fair value	2,765		$ 48,838
Equity securities redeemed for cash due to a third-party merger	(39,900)
Equity securities, FV-NI, Unrealized Gain (Loss)	(151)
Equtiy Securities without Readily Determinable Fair Values	1,702		1,753
U.S. Treasury inflation-protected securities held, at fair value	$ 436,200
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 53,500		68,000
Credit-related portion of impairments included in earnings, for securities on watch list	14,129	$ 14,245	14,199	$ 14,080
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	7,452,190		5,172,412
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	173,170		44,122
AFS securities in loss position at period-end, change in unrealized loss	129,000
Securities pledged as collateral	3,900,000
Securities pledged as collateral and may be sold or re-pledged	$ 553,200
No investment in a single issuer exceeds this percentage of stockholder's equity	10.00%
Number of investments in a single issuer that exceed 10% of stockholder's equity	0
State And Municipal Obligations [Member]
Investment [Line Items]
Auction rate securities held at fair value	$ 14,700
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	845,863		332,182
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	12,289		2,754
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	20,400
Credit-related portion of impairments included in earnings, for securities on watch list	14,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,055,292		730,830
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	18,211		$ 4,837
Private Equity Funds [Member]
Investment [Line Items]
Fair value adjustments, net	12,371
Equity Securities [Member]
Investment [Line Items]
Equity securities, FV-NI, Unrealized Gain	2,723	0
Adjustment to recognize dividend income on a liquidiated security	8,917	$ 0
Accounting Standards Update 2016-01 [Member]
Investment [Line Items]
Unrealized gain previously recorded in AOCI reclassified to Retained Earnings at adoption of ASU 2016-01	$ 33,320