Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 352,661	$ 98,188
Estimated Fair Value, 12 Months or More	481,177	2,399,747
Estimated Fair Value, Total	833,838	2,497,935
Gross Unrealized (Losses), Less than 12 Months	(1,347)	(382)
Gross Unrealized (Losses), 12 Months or More	(5,984)	(63,994)
Gross Unrealized (Losses), Total	(7,331)	(64,376)
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	6,585	0
Estimated Fair Value, 12 Months or More	12,886	25,045
Estimated Fair Value, Total	19,471	25,045
Gross Unrealized (Losses), Less than 12 Months	(105)	0
Gross Unrealized (Losses), 12 Months or More	(602)	(1,009)
Gross Unrealized (Losses), Total	(707)	(1,009)
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	4,954
Estimated Fair Value, 12 Months or More	6,216	12,802
Estimated Fair Value, Total	6,216	17,756
Gross Unrealized (Losses), Less than 12 Months	0	(9)
Gross Unrealized (Losses), 12 Months or More	(53)	(157)
Gross Unrealized (Losses), Total	(53)	(166)
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	23,544	9,163
Estimated Fair Value, 12 Months or More	18,529	61,141
Estimated Fair Value, Total	42,073	70,304
Gross Unrealized (Losses), Less than 12 Months	(107)	(54)
Gross Unrealized (Losses), 12 Months or More	(272)	(2,504)
Gross Unrealized (Losses), Total	(379)	(2,558)
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	112,879	31,931
Estimated Fair Value, 12 Months or More	278,120	731,749
Estimated Fair Value, Total	390,999	763,680
Gross Unrealized (Losses), Less than 12 Months	(230)	(172)
Gross Unrealized (Losses), 12 Months or More	(3,305)	(21,678)
Gross Unrealized (Losses), Total	(3,535)	(21,850)
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	46,643
Estimated Fair Value, 12 Months or More	151,271	1,296,221
Estimated Fair Value, Total	309,612	1,342,864
Gross Unrealized (Losses), Less than 12 Months	(738)	(110)
Gross Unrealized (Losses), 12 Months or More	(1,631)	(33,926)
Gross Unrealized (Losses), Total	(2,369)	(34,036)
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	51,312	5,497
Estimated Fair Value, 12 Months or More	14,155	272,789
Estimated Fair Value, Total	65,467	278,286
Gross Unrealized (Losses), Less than 12 Months	(167)	(37)
Gross Unrealized (Losses), 12 Months or More	(121)	(4,720)
Gross Unrealized (Losses), Total	$ (288)	$ (4,757)