Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	$ 567
Held-to-maturity, less than 12 months, gross unrealized losses	(2)
Held-to-maturity, 12 months or more, estimated fair value	27,862
Held-to-maturity, 12 months or more, gross unrealized losses	(274)
Held-to-maturity, total, estimated fair value	28,429
Held-to-maturity, total, gross unrealized losses	(276)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	52,072
Available-for-sale, less than 12 months, gross unrealized losses	(358)
Available-for-sale, 12 months or more, estimated fair value	1,303,292
Available-for-sale, 12 months or more, gross unrealized losses	(23,543)
Available-for-sale, total, estimated fair value	1,355,364
Available-for-sale, total, gross unrealized losses	(23,901)
U.S. Government agencies
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	0
Held-to-maturity, less than 12 months, gross unrealized losses	0
Held-to-maturity, 12 months or more, estimated fair value	12,987
Held-to-maturity, 12 months or more, gross unrealized losses	(9)
Held-to-maturity, total, estimated fair value	12,987
Held-to-maturity, total, gross unrealized losses	(9)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	5,583
Available-for-sale, less than 12 months, gross unrealized losses	(30)
Available-for-sale, 12 months or more, estimated fair value	124,129
Available-for-sale, 12 months or more, gross unrealized losses	(2,719)
Available-for-sale, total, estimated fair value	129,712
Available-for-sale, total, gross unrealized losses	(2,749)
Mortgage-backed securities
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	487
Held-to-maturity, less than 12 months, gross unrealized losses	(1)
Held-to-maturity, 12 months or more, estimated fair value	9,994
Held-to-maturity, 12 months or more, gross unrealized losses	(239)
Held-to-maturity, total, estimated fair value	10,481
Held-to-maturity, total, gross unrealized losses	(240)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	40,444
Available-for-sale, less than 12 months, gross unrealized losses	(248)
Available-for-sale, 12 months or more, estimated fair value	1,086,169
Available-for-sale, 12 months or more, gross unrealized losses	(19,594)
Available-for-sale, total, estimated fair value	1,126,613
Available-for-sale, total, gross unrealized losses	(19,842)
State and political subdivisions
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	80
Held-to-maturity, less than 12 months, gross unrealized losses	(1)
Held-to-maturity, 12 months or more, estimated fair value	4,881
Held-to-maturity, 12 months or more, gross unrealized losses	(26)
Held-to-maturity, total, estimated fair value	4,961
Held-to-maturity, total, gross unrealized losses	(27)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	1,016
Available-for-sale, less than 12 months, gross unrealized losses	(4)
Available-for-sale, 12 months or more, estimated fair value	92,894
Available-for-sale, 12 months or more, gross unrealized losses	(1,229)
Available-for-sale, total, estimated fair value	93,910
Available-for-sale, total, gross unrealized losses	(1,233)
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	5,029
Available-for-sale, less than 12 months, gross unrealized losses	(76)
Available-for-sale, 12 months or more, estimated fair value	100
Available-for-sale, 12 months or more, gross unrealized losses	(1)
Available-for-sale, total, estimated fair value	5,129
Available-for-sale, total, gross unrealized losses	$ (77)