Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 386,823	$ 98,188
Estimated Fair Value, 12 Months or More	518,078	2,399,747
Estimated Fair Value, Total	904,901	2,497,935
Gross Unrealized (Losses), Less than 12 Months	(1,088)	(382)
Gross Unrealized (Losses), 12 Months or More	(7,084)	(63,994)
Gross Unrealized (Losses), Total	(8,172)	(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	6,881	0
Estimated Fair Value, 12 Months or More	14,729	25,045
Estimated Fair Value, Total	21,610	25,045
Gross Unrealized (Losses), Less than 12 Months	(46)	0
Gross Unrealized (Losses), 12 Months or More	(691)	(1,009)
Gross Unrealized (Losses), Total	(737)	(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	(66)	(157)
Gross Unrealized (Losses), Total	(66)	(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	4,019	9,163
Estimated Fair Value, 12 Months or More	20,111	61,141
Estimated Fair Value, Total	24,130	70,304
Gross Unrealized (Losses), Less than 12 Months	(7)	(54)
Gross Unrealized (Losses), 12 Months or More	(301)	(2,504)
Gross Unrealized (Losses), Total	(308)	(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	163,210	31,931
Estimated Fair Value, 12 Months or More	299,860	731,749
Estimated Fair Value, Total	463,070	763,680
Gross Unrealized (Losses), Less than 12 Months	(302)	(172)
Gross Unrealized (Losses), 12 Months or More	(4,257)	(21,678)
Gross Unrealized (Losses), Total	(4,559)	(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	155,071	46,643
Estimated Fair Value, 12 Months or More	165,713	1,296,221
Estimated Fair Value, Total	320,784	1,342,864
Gross Unrealized (Losses), Less than 12 Months	(550)	(110)
Gross Unrealized (Losses), 12 Months or More	(1,677)	(33,926)
Gross Unrealized (Losses), Total	(2,227)	(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	57,642	5,497
Estimated Fair Value, 12 Months or More	11,449	272,789
Estimated Fair Value, Total	69,091	278,286
Gross Unrealized (Losses), Less than 12 Months	(183)	(37)
Gross Unrealized (Losses), 12 Months or More	(92)	(4,720)
Gross Unrealized (Losses), Total	$ (275)	$ (4,757)