Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	$ 132,542
Held-to-maturity, less than 12 months, gross unrealized losses	(1,805)
Held-to-maturity, 12 months or more, estimated fair value	48,841
Held-to-maturity, 12 months or more, gross unrealized losses	(1,032)
Held-to-maturity, total, estimated fair value	181,383
Held-to-maturity, total, gross unrealized losses	(2,837)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	825,604
Available-for-sale, less than 12 months, gross unrealized losses	(16,668)
Available-for-sale, 12 months or more, estimated fair value	988,819
Available-for-sale, 12 months or more, gross unrealized losses	(49,173)
Available-for-sale, total, estimated fair value	1,814,423
Available-for-sale, total, gross unrealized losses	(65,841)
U.S. Government agencies
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	3,972
Held-to-maturity, less than 12 months, gross unrealized losses	(11)
Held-to-maturity, 12 months or more, estimated fair value	30,898
Held-to-maturity, 12 months or more, gross unrealized losses	(102)
Held-to-maturity, total, estimated fair value	34,870
Held-to-maturity, total, gross unrealized losses	(113)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	56,785
Available-for-sale, less than 12 months, gross unrealized losses	(689)
Available-for-sale, 12 months or more, estimated fair value	84,675
Available-for-sale, 12 months or more, gross unrealized losses	(3,659)
Available-for-sale, total, estimated fair value	141,460
Available-for-sale, total, gross unrealized losses	(4,348)
Mortgage-backed securities
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	3,830
Held-to-maturity, less than 12 months, gross unrealized losses	(106)
Held-to-maturity, 12 months or more, estimated fair value	9,404
Held-to-maturity, 12 months or more, gross unrealized losses	(592)
Held-to-maturity, total, estimated fair value	13,234
Held-to-maturity, total, gross unrealized losses	(698)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	586,783
Available-for-sale, less than 12 months, gross unrealized losses	(11,469)
Available-for-sale, 12 months or more, estimated fair value	808,969
Available-for-sale, 12 months or more, gross unrealized losses	(40,719)
Available-for-sale, total, estimated fair value	1,395,752
Available-for-sale, total, gross unrealized losses	(52,188)
State and political subdivisions
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	123,574
Held-to-maturity, less than 12 months, gross unrealized losses	(1,682)
Held-to-maturity, 12 months or more, estimated fair value	8,539
Held-to-maturity, 12 months or more, gross unrealized losses	(338)
Held-to-maturity, total, estimated fair value	132,113
Held-to-maturity, total, gross unrealized losses	(2,020)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	182,036
Available-for-sale, less than 12 months, gross unrealized losses	(4,510)
Available-for-sale, 12 months or more, estimated fair value	95,076
Available-for-sale, 12 months or more, gross unrealized losses	(4,794)
Available-for-sale, total, estimated fair value	277,112
Available-for-sale, total, gross unrealized losses	(9,304)
Other securities
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	1,166
Held-to-maturity, less than 12 months, gross unrealized losses	(6)
Held-to-maturity, 12 months or more, estimated fair value	0
Held-to-maturity, 12 months or more, gross unrealized losses	0
Held-to-maturity, total, estimated fair value	1,166
Held-to-maturity, total, gross unrealized losses	(6)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	0
Available-for-sale, less than 12 months, gross unrealized losses	0
Available-for-sale, 12 months or more, estimated fair value	99
Available-for-sale, 12 months or more, gross unrealized losses	(1)
Available-for-sale, total, estimated fair value	99
Available-for-sale, total, gross unrealized losses	$ (1)