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Investment Securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months $ 6,531,856 $ 8,458,946
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (88,744) (123,747)
Estimated fair value, 12 months or more 150,376 169,469
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (12,047) (17,768)
Held to maturity, Estimated fair value, Less than 12 months 1,055,438 654,731
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (11,392) (8,190)
Held to maturity, Estimated fair value, 12 months or more 85,587 86,388
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (33,891) (36,528)
Total investment securities, fair value less than 12 months 7,587,294 9,113,677
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses (100,136) (131,937)
Total of investment securities, fair value, 12 Months or More 235,963 255,857
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses (45,938) (54,296)
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 1,952,450 1,710,241
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (10,838) (9,950)
Estimated fair value, 12 months or more   2,295
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses   (2)
Obligations of States and Political Subdivisions [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, 12 months or more 474 593
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (2) (6)
Held to maturity, Estimated fair value, Less than 12 months 8,698 17,988
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (68) (126)
Held to maturity, Estimated fair value, 12 months or more 8,496 11,891
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (57) (98)
Government Issued or Guaranteed [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 4,560,342 6,730,829
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (77,584) (113,374)
Estimated fair value, 12 months or more 89,307 81,003
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (841) (615)
Held to maturity, Estimated fair value, Less than 12 months 1,046,740 618,832
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (11,324) (6,842)
Held to maturity, Estimated fair value, 12 months or more 18,420 17,481
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (562) (532)
Privately Issued Mortgage-Backed Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, 12 months or more 19 27
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (1) (1)
Other Debt Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 998 100
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (2) (1)
Estimated fair value, 12 months or more 60,422 85,400
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (11,057) (16,995)
Equity Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 18,066 17,776
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (320) (422)
Estimated fair value, 12 months or more 154 151
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (146) (149)
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   17,911
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses   (1,222)
Held to maturity, Estimated fair value, 12 months or more 58,671 57,016
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses $ (33,272) $ (35,898)