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Investment Securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months $ 4,072,824 $ 2,387,341
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (71,086) (15,482)
Estimated fair value, 12 months or more 4,788,658 4,887,991
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (163,469) (112,184)
Held to maturity, Estimated fair value, Less than 12 months 2,282,509 1,339,774
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (46,289) (8,256)
Held to maturity, Estimated fair value, 12 months or more 309,203 327,060
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (36,412) (33,114)
Total investment securities, fair value less than 12 months 6,355,333 3,727,115
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses (117,375) (23,738)
Total of investment securities, fair value, 12 Months or More 5,097,861 5,215,051
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses (199,881) (145,298)
U.S. Treasury and Federal Agencies [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 241,594 278,132
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (2,314) (1,761)
Estimated fair value, 12 months or more 1,699,784 1,669,355
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (18,368) (16,417)
Obligations of States and Political Subdivisions [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 1,020  
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (1)  
Estimated fair value, 12 months or more 376 474
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (1) (2)
Held to maturity, Estimated fair value, Less than 12 months 1,891 2,954
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (4) (4)
Held to maturity, Estimated fair value, 12 months or more 5,454 6,110
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (32) (45)
Government Issued or Guaranteed [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 3,825,869 2,106,142
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (68,681) (13,695)
Estimated fair value, 12 months or more 3,023,992 3,138,841
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (137,984) (84,892)
Held to maturity, Estimated fair value, Less than 12 months 2,280,618 1,331,759
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (46,285) (7,036)
Held to maturity, Estimated fair value, 12 months or more 249,897 265,695
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (12,296) (6,710)
Other Debt Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 4,341 3,067
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (90) (26)
Estimated fair value, 12 months or more 64,506 61,159
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (7,116) (10,449)
Equity Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, 12 months or more   18,162
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses   (424)
Privately Issued [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Held to maturity, Estimated fair value, Less than 12 months   5,061
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses   (1,216)
Held to maturity, Estimated fair value, 12 months or more 53,852 55,255
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses $ (24,084) $ (26,359)