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, 2018	Dec. 31, 2017
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 889,892	$ 1,554,184
Estimated Fair Value, 12 Months or More	1,758,673	1,022,055
Estimated Fair Value, Total	2,648,565	2,576,239
Gross Unrealized (Losses), Less than 12 Months	(22,238)	(14,150)
Gross Unrealized (Losses), 12 Months or More	(85,883)	(27,986)
Gross Unrealized (Losses), Total	(108,121)	(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	26,421	29,432
Estimated Fair Value, Total	26,421	34,646
Gross Unrealized (Losses), Less than 12 Months	0	(113)
Gross Unrealized (Losses), 12 Months or More	(1,266)	(687)
Gross Unrealized (Losses), Total	(1,266)	(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	29,786	19,345
Estimated Fair Value, 12 Months or More	3,791	3,874
Estimated Fair Value, Total	33,577	23,219
Gross Unrealized (Losses), Less than 12 Months	(179)	(80)
Gross Unrealized (Losses), 12 Months or More	(41)	(33)
Gross Unrealized (Losses), Total	(220)	(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	24,843	37,304
Estimated Fair Value, 12 Months or More	49,000	29,446
Estimated Fair Value, Total	73,843	66,750
Gross Unrealized (Losses), Less than 12 Months	(677)	(351)
Gross Unrealized (Losses), 12 Months or More	(2,827)	(825)
Gross Unrealized (Losses), Total	(3,504)	(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	134,720	506,410
Estimated Fair Value, 12 Months or More	655,161	369,060
Estimated Fair Value, Total	789,881	875,470
Gross Unrealized (Losses), Less than 12 Months	(3,928)	(4,219)
Gross Unrealized (Losses), 12 Months or More	(30,048)	(8,589)
Gross Unrealized (Losses), Total	(33,976)	(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	531,793	755,013
Estimated Fair Value, 12 Months or More	865,762	534,955
Estimated Fair Value, Total	1,397,555	1,289,968
Gross Unrealized (Losses), Less than 12 Months	(14,120)	(7,668)
Gross Unrealized (Losses), 12 Months or More	(46,065)	(16,591)
Gross Unrealized (Losses), Total	(60,185)	(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	168,750	230,898
Estimated Fair Value, 12 Months or More	158,538	55,288
Estimated Fair Value, Total	327,288	286,186
Gross Unrealized (Losses), Less than 12 Months	(3,334)	(1,719)
Gross Unrealized (Losses), 12 Months or More	(5,636)	(1,261)
Gross Unrealized (Losses), Total	$ (8,970)	$ (2,980)