Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 29,032	$ 352,661
Estimated Fair Value, 12 Months or More	76,126	481,177
Estimated Fair Value, Total	105,158	833,838
Gross Unrealized (Losses), Less than 12 Months	(177)	(1,347)
Gross Unrealized (Losses), 12 Months or More	(1,411)	(5,984)
Gross Unrealized (Losses), Total	(1,588)	(7,331)
U.S. Government Agency Obligations [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,245	6,585
Estimated Fair Value, 12 Months or More	12,708	12,886
Estimated Fair Value, Total	15,953	19,471
Gross Unrealized (Losses), Less than 12 Months	(23)	(105)
Gross Unrealized (Losses), 12 Months or More	(569)	(602)
Gross Unrealized (Losses), Total	(592)	(707)
Obligations of States and Political Subdivisions [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	0	0
Estimated Fair Value, 12 Months or More	6,217	6,216
Estimated Fair Value, Total	6,217	6,216
Gross Unrealized (Losses), Less than 12 Months	0	0
Gross Unrealized (Losses), 12 Months or More	(41)	(53)
Gross Unrealized (Losses), Total	(41)	(53)
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	65	23,544
Estimated Fair Value, 12 Months or More	774	18,529
Estimated Fair Value, Total	839	42,073
Gross Unrealized (Losses), Less than 12 Months	(1)	(107)
Gross Unrealized (Losses), 12 Months or More	(2)	(272)
Gross Unrealized (Losses), Total	(3)	(379)
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	17,254	112,879
Estimated Fair Value, 12 Months or More	55,755	278,120
Estimated Fair Value, Total	73,009	390,999
Gross Unrealized (Losses), Less than 12 Months	(136)	(230)
Gross Unrealized (Losses), 12 Months or More	(789)	(3,305)
Gross Unrealized (Losses), Total	(925)	(3,535)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	8,468	51,312
Estimated Fair Value, 12 Months or More	672	14,155
Estimated Fair Value, Total	9,140	65,467
Gross Unrealized (Losses), Less than 12 Months	(17)	(167)
Gross Unrealized (Losses), 12 Months or More	(10)	(121)
Gross Unrealized (Losses), Total	$ (27)	(288)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months		158,341
Estimated Fair Value, 12 Months or More		151,271
Estimated Fair Value, Total		309,612
Gross Unrealized (Losses), Less than 12 Months		(738)
Gross Unrealized (Losses), 12 Months or More		(1,631)
Gross Unrealized (Losses), Total		$ (2,369)