Investment Securities (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment [Line Items]
Non-marketable FHLB and FRB stock held for debt and regulatory purposes	$ 46,900	$ 46,800
Non-marketable private equity investments held	64,700	65,600
U.S. Treasury inflation-protected securities held, at fair value	$ 400,500
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 78,900	95,800
Credit-related portion of impairments included in earnings, for securities on watch list	$ 14,362	$ 14,129	$ 14,166	$ 13,734
Number of individual securities held in the available for sale portfolio | Securities	2,000
Securities in Unrealized Loss Positions, Number of Positions | Securities	145	466
Fair value of securities in an unrealized loss position	$ 877,439	$ 5,120,187
AFS securities in loss position at period-end, change in unrealized loss	(38,400)
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 14,245	52,620
Minimum period that OTTI securities have been in a loss position	12 months
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 312,122	767,349
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Percent of Portfolio	3.40%
Securities pledged as collateral	$ 4,600,000
Securities pledged as collateral and may be sold or re-pledged	$ 583,200
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%
State And Municipal Obligations [Member]
Investment [Line Items]
Fair value of securities in an unrealized loss position	$ 26,853	98,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	692	1,739
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,478	31,120
Available-for-sale Securities, Excluding Auction Rate Securities	$ 1,800,000
State and municipal obligation portfolio, excluding auction rate securities, as a percentage of total available for sale securities	19.90%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 44,800
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	37,100
Credit-related portion of impairments included in earnings, for securities on watch list	14,400
Fair value of securities in an unrealized loss position	61,201	708,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	104	7,819
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 57,411	$ 54,536