Investment Securities (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) Securities	Dec. 31, 2014 USD ($) Securities	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Investment [Line Items]
Non-marketable FHLB and FRB stock held for debt and regulatory purposes	$ 46,800	$ 46,600
Non-marketable private equity investments held	69,500	60,200
U.S. Treasury inflation-protected securities held	$ 420,300
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 102,800	123,900
Credit-related portion of impairments included in earnings, for securities on watch list	$ 14,144	$ 13,734	$ 13,750	$ 12,499
Number of individual securities held in the available for sale portfolio | Securities	2,000
Securities in Unrealized Loss Positions, Number of Positions | Securities	332	363
Fair value of securities in an unrealized loss position	$ 2,521,993	$ 3,262,858
AFS securities in loss position at period-end, change in unrealized loss	5,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 29,918	35,384
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	$ 789,751	1,137,135
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Percent of Portfolio	8.30%
Securities pledged as collateral	$ 4,300,000
Securities pledged as collateral and may be sold or re-pledged	$ 364,100
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]
Auction rate securities held at fair value	$ 93,200
Fair value of securities in an unrealized loss position	230,648	388,682
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6,493	8,354
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	110,550	215,702
Unrealized losses included in auction rate securities held	5,500
Unrealized losses included in other state and municipal obligations held	1,000
Aggregate fair value of state and municipal obligations 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	18.50%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 39,400
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	47,400
Credit-related portion of impairments included in earnings, for securities on watch list	14,100
Fair value of securities in an unrealized loss position	253,640	185,314
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	648	1,228
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 53,765	$ 43,659