Investment Securities (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Securities Investments_In_Single_Issuer	Dec. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment [Line Items]
Non-marketable FHLB and FRB stock held for debt and regulatory purposes	$ 43,300	$ 46,900
Non-marketable private equity investments held	57,200	52,300
U.S. Treasury inflation-protected securities held	$ 442,100
Percentage decrease requiring a review for impairment	20.00%
Fair value of securities on other-than-temporary impairment watch list	$ 68,000	79,600
Cumulative, credit-related portion of impairment (recorded in earnings) on the certain non-agency mortgage-backed securities on the watch list	$ 14,199	$ 14,080	$ 14,129	$ 13,734
Number of individual securities held in the available for sale portfolio | Securities	2,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities	635	771
Portfolio securities aggregate fair value loss position	$ 5,172,412	$ 4,480,405
Available for sale securities in loss position at period end, change in unrealized loss	(16,800)
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	44,122	60,942
Fair value of securities in loss position for twelve months or longer	$ 1,156,715	444,252
Fair value of securities in a loss position for 12 months or longer, as a percentage of total available for sale portfolio	13.20%
Available for sale securities	$ 8,774,280	9,649,203
Securities pledged as collateral	3,800,000	4,400,000
Securities pledged and may be sold or re-pledged	$ 662,500
Number of investments in a single issuer that exceed 10% of stockholder's equity | Investments_In_Single_Issuer	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	$ 17,000
Portfolio securities aggregate fair value loss position	332,182	715,974
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,754	12,693
Fair value of securities in loss position for twelve months or longer	49,339	15,195
Available for sale securities	$ 1,611,366	1,778,214
State and municipal obligation portfolio, as a percentage of total available for sale securities	18.40%
Equity securities [Member]
Investment [Line Items]
Common stock classified as equity securities held by the holding company	$ 45,900
Available for sale securities	48,838	51,263
Non-agency mortgage-backed securities [Member]
Investment [Line Items]
Fair value of securities on other-than-temporary impairment watch list	26,200
Cumulative, credit-related portion of impairment (recorded in earnings) on the certain non-agency mortgage-backed securities on the watch list	14,200
Portfolio securities aggregate fair value loss position	730,830	723,077
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4,837	6,493
Fair value of securities in loss position for twelve months or longer	153,813	34,946
Available for sale securities	$ 905,793	$ 1,055,639