Investment Securities (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment [Line Items]
Non-marketable FHLB and FRB stock held for debt and regulatory purposes	$ 47,100	$ 46,900
Non-marketable private equity investments held	54,300	52,300
U.S. Treasury inflation-protected securities held, at fair value	$ 434,900
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 74,200	79,600
Credit-related portion of impairments included in earnings, for securities on watch list	$ 14,116	$ 14,080	$ 14,234	$ 14,129
Number of individual securities held in the available for sale portfolio | Securities	2,000
Securities in Unrealized Loss Positions, Number of Positions | Securities	535	771
Fair value of securities in an unrealized loss position	$ 3,710,987	$ 4,480,405
AFS securities in loss position at period-end, change in unrealized loss	(16,600)
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	44,319	60,942
Available-for-sale Securities	$ 9,671,975	9,649,203
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	$ 330,027	444,252
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Percent of Portfolio	3.40%
Securities pledged as collateral	$ 3,800,000
Securities pledged as collateral and may be sold or re-pledged	$ 720,100
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	$ 17,100
Fair value of securities in an unrealized loss position	338,822	715,974
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6,288	12,693
Unrealized Losses on Auction Rate Securities	392
Available-for-sale Securities	1,796,037	1,778,214
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 10,051	15,195
State and municipal obligation portfolio, excluding auction rate securities, as a percentage of total available for sale securities	18.60%
Equity Securities [Member]
Investment [Line Items]
Common stock held by the holding company	$ 53,800
Available-for-sale Securities	53,865	51,263
Non-Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	29,300
Credit-related portion of impairments included in earnings, for securities on watch list	14,100
Fair value of securities in an unrealized loss position	709,298	723,077
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6,550	6,493
Available-for-sale Securities	1,078,256	1,055,639
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 28,136	$ 34,946