Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 44,675	$ 98,188
Estimated Fair Value, 12 Months or More	1,092,728	2,399,747
Estimated Fair Value, Total	1,137,403	2,497,935
Gross Unrealized (Losses), Less than 12 Months	(219)	(382)
Gross Unrealized (Losses), 12 Months or More	(11,735)	(63,994)
Gross Unrealized (Losses), Total	(11,954)	(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	645	0
Estimated Fair Value, 12 Months or More	22,179	25,045
Estimated Fair Value, Total	22,824	25,045
Gross Unrealized (Losses), Less than 12 Months	(1)	0
Gross Unrealized (Losses), 12 Months or More	(789)	(1,009)
Gross Unrealized (Losses), Total	(790)	(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	8,020	12,802
Estimated Fair Value, Total	8,020	17,756
Gross Unrealized (Losses), Less than 12 Months	0	(9)
Gross Unrealized (Losses), 12 Months or More	(88)	(157)
Gross Unrealized (Losses), Total	(88)	(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	299	9,163
Estimated Fair Value, 12 Months or More	45,194	61,141
Estimated Fair Value, Total	45,493	70,304
Gross Unrealized (Losses), Less than 12 Months	(3)	(54)
Gross Unrealized (Losses), 12 Months or More	(705)	(2,504)
Gross Unrealized (Losses), Total	(708)	(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	2,817	31,931
Estimated Fair Value, 12 Months or More	555,445	731,749
Estimated Fair Value, Total	558,262	763,680
Gross Unrealized (Losses), Less than 12 Months	(10)	(172)
Gross Unrealized (Losses), 12 Months or More	(5,796)	(21,678)
Gross Unrealized (Losses), Total	(5,806)	(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	9,392	46,643
Estimated Fair Value, 12 Months or More	403,393	1,296,221
Estimated Fair Value, Total	412,785	1,342,864
Gross Unrealized (Losses), Less than 12 Months	(75)	(110)
Gross Unrealized (Losses), 12 Months or More	(3,981)	(33,926)
Gross Unrealized (Losses), Total	(4,056)	(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	31,522	5,497
Estimated Fair Value, 12 Months or More	58,497	272,789
Estimated Fair Value, Total	90,019	278,286
Gross Unrealized (Losses), Less than 12 Months	(130)	(37)
Gross Unrealized (Losses), 12 Months or More	(376)	(4,720)
Gross Unrealized (Losses), Total	$ (506)	$ (4,757)