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Investment Securities (Narrative) (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 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 55,000 52,300    
U.S. Treasury inflation-protected securities held, at fair value $ 432,000      
Percentage decrease requiring a review for impairment 20.00%      
Fair value of securities on other-than-temporary impairment watch list $ 68,900 79,600    
Credit-related portion of impairments included in earnings, for securities on watch list 14,254 14,080 $ 14,362 $ 14,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 31,951 60,942    
Available-for-sale Securities 9,439,701 9,649,203    
Securities pledged as collateral 3,900,000      
Securities pledged as collateral and may be sold or re-pledged $ 719,800      
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 $ 16,800      
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 1,988 12,693    
Unrealized Losses on Auction Rate Securities 230      
Available-for-sale Securities $ 1,777,797 1,778,214    
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 $ 47,700      
Available-for-sale Securities 47,847 51,263    
Non-Agency Mortgage-Backed Securities [Member]        
Investment [Line Items]        
Fair value of securities on other-than-temporary impairment watch list 30,300      
Credit-related portion of impairments included in earnings, for securities on watch list 14,300      
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 4,410 6,493    
Available-for-sale Securities $ 1,066,889 $ 1,055,639