Investment Securities (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities	Sep. 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	61,000	65,600
U.S. Treasury inflation-protected securities held, at fair value	$ 437,900
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 103,700	95,800
Credit-related portion of impairments included in earnings, for securities on watch list	$ 14,228	$ 14,129	$ 14,144	$ 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	152	466
Fair value of securities in an unrealized loss position	$ 955,010	$ 5,120,187
AFS securities in loss position at period-end, change in unrealized loss	(41,700)
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 10,954	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	$ 469,123	767,349
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Percent of Portfolio	5.00%
Securities pledged as collateral	$ 4,000,000
Securities pledged as collateral and may be sold or re-pledged	$ 578,100
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	$ 42,611	98,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	616	1,739
Auction Rate Securities Available For Sale	378
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	16,276	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	19.50%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 46,900
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	34,600
Credit-related portion of impairments included in earnings, for securities on watch list	14,200
Fair value of securities in an unrealized loss position	100,353	708,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	239	7,819
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 39,070	$ 54,536