Investment Securities (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities	Mar. 31, 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	70,000	65,600
U.S. Treasury inflation-protected securities held	$ 397,400
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 85,900	95,800
Credit-related portion of impairments included in earnings, for securities on watch list	$ 14,234	$ 14,129	$ 13,722	$ 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	228	466
Fair value of securities in an unrealized loss position	$ 1,556,291	$ 5,120,187
AFS securities in loss position at period-end, change in unrealized loss	(33,500)
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 19,129	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	$ 314,279	767,349
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Percent of Portfolio	3.30%
Securities pledged as collateral	$ 3,900,000
Securities pledged as collateral and may be sold or re-pledged	$ 581,000
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	$ 42,621	98,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,243	1,739
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,997	31,120
Unrealized losses included in auction rate securities held	1,100
Unrealized losses included in other state and municipal obligations held	171
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	18.80%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 43,000
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	39,900
Credit-related portion of impairments included in earnings, for securities on watch list	14,200
Fair value of securities in an unrealized loss position	221,948	708,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,242	7,819
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 66,103	$ 54,536