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Investment Securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months $ 7,331,466 $ 8,458,946
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (118,396) (123,747)
Estimated fair value, 12 months or more 171,883 169,469
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (16,109) (17,768)
Held to maturity, Estimated fair value, Less than 12 months 916,922 654,731
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (10,129) (8,190)
Held to maturity, Estimated fair value, 12 months or more 101,518 86,388
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (36,306) (36,528)
Total investment securities, fair value less than 12 months 8,248,388 9,113,677
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses (128,525) (131,937)
Total of investment securities, fair value, 12 Months or More 273,401 255,857
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses (52,415) (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 1,745,277 1,710,241
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (9,494) (9,950)
Estimated fair value, 12 months or more 2,044 2,295
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (10) (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 472 593
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (4) (6)
Held to maturity, Estimated fair value, Less than 12 months 13,205 17,988
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (87) (126)
Held to maturity, Estimated fair value, 12 months or more 9,909 11,891
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (69) (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 5,549,268 6,730,829
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (108,380) (113,374)
Estimated fair value, 12 months or more 93,425 81,003
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (737) (615)
Held to maturity, Estimated fair value, Less than 12 months 896,556 618,832
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (10,013) (6,842)
Held to maturity, Estimated fair value, 12 months or more 17,472 17,481
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (564) (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 25 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 19,052 100
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (100) (1)
Estimated fair value, 12 months or more 75,763 85,400
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (15,211) (16,995)
Equity Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 17,869 17,776
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (422) (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 7,161 17,911
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (29) (1,222)
Held to maturity, Estimated fair value, 12 months or more 74,137 57,016
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses $ (35,673) $ (35,898)