Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 235,300
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	668
Held to Maturity, More than 12 Months, Estimated Fair Value	45,882
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,809
Held to Maturity, Total Estimated Fair Value	281,182
Held-to-maturity Securities, Gross Unrealized Losses	2,477	$ 2,784
Available for Sale, Less than 12 Months, Estimated Fair Value	4,987,983
Available for Sale, Less than 12 Months, Unrealized Losses	65,109
Available for Sale, More than 12 Months, Estimated Fair Value	1,441,043
Available for Sale, More than 12 Months, Unrealized Losses	51,783
Available for Sale, Total Estimated Fair Value	6,429,026
Available-for-sale Securities, Gross Unrealized Loss, before Tax	116,892	34,777
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,103
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	25
Held to Maturity, More than 12 Months, Estimated Fair Value	1,392
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	48
Held to Maturity, Total Estimated Fair Value	2,495
Held-to-maturity Securities, Gross Unrealized Losses	73	38
Available for Sale, Less than 12 Months, Estimated Fair Value	238,548
Available for Sale, Less than 12 Months, Unrealized Losses	4,687
Available for Sale, More than 12 Months, Estimated Fair Value	58,923
Available for Sale, More than 12 Months, Unrealized Losses	3,092
Available for Sale, Total Estimated Fair Value	297,471
Available-for-sale Securities, Gross Unrealized Loss, before Tax	7,779	2,250
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	232,707
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	633
Held to Maturity, More than 12 Months, Estimated Fair Value	44,490
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,761
Held to Maturity, Total Estimated Fair Value	277,197
Held-to-maturity Securities, Gross Unrealized Losses	2,394	2,746
Available for Sale, Less than 12 Months, Estimated Fair Value	1,907,610
Available for Sale, Less than 12 Months, Unrealized Losses	25,535
Available for Sale, More than 12 Months, Estimated Fair Value	813,864
Available for Sale, More than 12 Months, Unrealized Losses	39,272
Available for Sale, Total Estimated Fair Value	2,721,474
Available-for-sale Securities, Gross Unrealized Loss, before Tax	64,807	16,795
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,490
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	1,490
Held-to-maturity Securities, Gross Unrealized Losses	10	0
Available-for-sale Securities, Gross Unrealized Loss, before Tax	0	0
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	2,841,825
Available for Sale, Less than 12 Months, Unrealized Losses	34,887
Available for Sale, More than 12 Months, Estimated Fair Value	568,256
Available for Sale, More than 12 Months, Unrealized Losses	9,419
Available for Sale, Total Estimated Fair Value	3,410,081
Available-for-sale Securities, Gross Unrealized Loss, before Tax	$ 44,306	$ 15,732