Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 8,862
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	53
Held to Maturity, More than 12 Months, Estimated Fair Value	74,965
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,528
Held to Maturity, Total Estimated Fair Value	83,827
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,581	$ 6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	1,902,220
Available for Sale, Less than 12 Months, Unrealized Losses	13,381
Available for Sale, More than 12 Months, Estimated Fair Value	862,209
Available for Sale, More than 12 Months, Unrealized Losses	31,474
Available for Sale, Total Estimated Fair Value	2,764,429
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	44,855	108,372
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	2,212
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	24
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	2,212
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	24	0
Available for Sale, Less than 12 Months, Estimated Fair Value	75,441
Available for Sale, Less than 12 Months, Unrealized Losses	618
Available for Sale, More than 12 Months, Estimated Fair Value	19,458
Available for Sale, More than 12 Months, Unrealized Losses	686
Available for Sale, Total Estimated Fair Value	94,899
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,304	1,293
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	5,301
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	28
Held to Maturity, More than 12 Months, Estimated Fair Value	74,965
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,528
Held to Maturity, Total Estimated Fair Value	80,266
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2,556	6,335
Available for Sale, Less than 12 Months, Estimated Fair Value	986,705
Available for Sale, Less than 12 Months, Unrealized Losses	9,713
Available for Sale, More than 12 Months, Estimated Fair Value	842,751
Available for Sale, More than 12 Months, Unrealized Losses	30,788
Available for Sale, Total Estimated Fair Value	1,829,456
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	40,501	98,134
Other Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	1,349
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
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	1,349
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	1	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Available for Sale, Less than 12 Months, Estimated Fair Value	840,074
Available for Sale, Less than 12 Months, Unrealized Losses	3,050
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	840,074
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 3,050	$ 8,945