Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	$ 106,205
Held-to-maturity, less than 12 months, gross unrealized losses	(1,298)
Held-to-maturity, 12 months or mote, estimated fair value	40,113
Held-to-maturity, 12 months or more, gross unrealized losses	(664)
Held-to-maturity, total, estimated fair value	146,318
Held-to-maturity, total, gross unrealized losses	(1,962)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	941,104
Available-for-sale, less than 12 months, gross unrealized losses	(17,990)
Available-for-sale, 12 months or more, estimated fair value	735,589
Available-for-sale, 12 months or more, gross unrealized losses	(35,118)
Available-for-sale, total, estimated fair value	1,676,693
Available-for-sale, total, gross unrealized losses	(53,108)
U.S. Government agencies
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	5,963
Held-to-maturity, less than 12 months, gross unrealized losses	(12)
Held-to-maturity, 12 months or mote, estimated fair value	30,827
Held-to-maturity, 12 months or more, gross unrealized losses	(174)
Held-to-maturity, total, estimated fair value	36,790
Held-to-maturity, total, gross unrealized losses	(186)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	113,594
Available-for-sale, less than 12 months, gross unrealized losses	(1,753)
Available-for-sale, 12 months or more, estimated fair value	32,174
Available-for-sale, 12 months or more, gross unrealized losses	(2,073)
Available-for-sale, total, estimated fair value	145,768
Available-for-sale, total, gross unrealized losses	(3,826)
Mortgage-backed securities
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	6,106
Held-to-maturity, less than 12 months, gross unrealized losses	(148)
Held-to-maturity, 12 months or mote, estimated fair value	7,934
Held-to-maturity, 12 months or more, gross unrealized losses	(455)
Held-to-maturity, total, estimated fair value	14,040
Held-to-maturity, total, gross unrealized losses	(603)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	685,406
Available-for-sale, less than 12 months, gross unrealized losses	(14,319)
Available-for-sale, 12 months or more, estimated fair value	630,291
Available-for-sale, 12 months or more, gross unrealized losses	(29,440)
Available-for-sale, total, estimated fair value	1,315,697
Available-for-sale, total, gross unrealized losses	(43,759)
State and political subdivisions
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	94,136
Held-to-maturity, less than 12 months, gross unrealized losses	(1,138)
Held-to-maturity, 12 months or mote, estimated fair value	1,352
Held-to-maturity, 12 months or more, gross unrealized losses	(35)
Held-to-maturity, total, estimated fair value	95,488
Held-to-maturity, total, gross unrealized losses	(1,173)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	142,104
Available-for-sale, less than 12 months, gross unrealized losses	(1,918)
Available-for-sale, 12 months or more, estimated fair value	73,025
Available-for-sale, 12 months or more, gross unrealized losses	(3,603)
Available-for-sale, total, estimated fair value	215,129
Available-for-sale, total, gross unrealized losses	(5,521)
Other securities
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	(2)
Available-for-sale, total, estimated fair value	99
Available-for-sale, total, gross unrealized losses	$ (2)