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, 2018	Dec. 31, 2017
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 98,188	$ 1,554,184
Estimated Fair Value, 12 Months or More	2,399,747	1,022,055
Estimated Fair Value, Total	2,497,935	2,576,239
Gross Unrealized (Losses), Less than 12 Months	(382)	(14,150)
Gross Unrealized (Losses), 12 Months or More	(63,994)	(27,986)
Gross Unrealized (Losses), Total	(64,376)	(42,136)
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	0	5,214
Estimated Fair Value, 12 Months or More	25,045	29,432
Estimated Fair Value, Total	25,045	34,646
Gross Unrealized (Losses), Less than 12 Months		(113)
Gross Unrealized (Losses), 12 Months or More	(1,009)	(687)
Gross Unrealized (Losses), Total	(1,009)	(800)
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	4,954	19,345
Estimated Fair Value, 12 Months or More	12,802	3,874
Estimated Fair Value, Total	17,756	23,219
Gross Unrealized (Losses), Less than 12 Months	(9)	(80)
Gross Unrealized (Losses), 12 Months or More	(157)	(33)
Gross Unrealized (Losses), Total	(166)	(113)
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	9,163	37,304
Estimated Fair Value, 12 Months or More	61,141	29,446
Estimated Fair Value, Total	70,304	66,750
Gross Unrealized (Losses), Less than 12 Months	(54)	(351)
Gross Unrealized (Losses), 12 Months or More	(2,504)	(825)
Gross Unrealized (Losses), Total	(2,558)	(1,176)
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	31,931	506,410
Estimated Fair Value, 12 Months or More	731,749	369,060
Estimated Fair Value, Total	763,680	875,470
Gross Unrealized (Losses), Less than 12 Months	(172)	(4,219)
Gross Unrealized (Losses), 12 Months or More	(21,678)	(8,589)
Gross Unrealized (Losses), Total	(21,850)	(12,808)
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	46,643	755,013
Estimated Fair Value, 12 Months or More	1,296,221	534,955
Estimated Fair Value, Total	1,342,864	1,289,968
Gross Unrealized (Losses), Less than 12 Months	(110)	(7,668)
Gross Unrealized (Losses), 12 Months or More	(33,926)	(16,591)
Gross Unrealized (Losses), Total	(34,036)	(24,259)
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	5,497	230,898
Estimated Fair Value, 12 Months or More	272,789	55,288
Estimated Fair Value, Total	278,286	286,186
Gross Unrealized (Losses), Less than 12 Months	(37)	(1,719)
Gross Unrealized (Losses), 12 Months or More	(4,720)	(1,261)
Gross Unrealized (Losses), Total	$ (4,757)	$ (2,980)