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, 2018	Dec. 31, 2017
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,804,223	$ 1,554,184
Estimated Fair Value, 12 Months or More	945,636	1,022,055
Estimated Fair Value, Total	2,749,859	2,576,239
Gross Unrealized (Losses), Less than 12 Months	(47,799)	(14,150)
Gross Unrealized (Losses), 12 Months or More	(46,298)	(27,986)
Gross Unrealized (Losses), Total	(94,097)	(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	5,011	5,214
Estimated Fair Value, 12 Months or More	23,844	29,432
Estimated Fair Value, Total	28,855	34,646
Gross Unrealized (Losses), Less than 12 Months	(227)	(113)
Gross Unrealized (Losses), 12 Months or More	(904)	(687)
Gross Unrealized (Losses), Total	(1,131)	(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	12,932	19,345
Estimated Fair Value, 12 Months or More	3,546	3,874
Estimated Fair Value, Total	16,478	23,219
Gross Unrealized (Losses), Less than 12 Months	(103)	(80)
Gross Unrealized (Losses), 12 Months or More	(44)	(33)
Gross Unrealized (Losses), Total	(147)	(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	33,409	37,304
Estimated Fair Value, 12 Months or More	32,821	29,446
Estimated Fair Value, Total	66,230	66,750
Gross Unrealized (Losses), Less than 12 Months	(1,091)	(351)
Gross Unrealized (Losses), 12 Months or More	(1,826)	(825)
Gross Unrealized (Losses), Total	(2,917)	(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	503,290	506,410
Estimated Fair Value, 12 Months or More	329,200	369,060
Estimated Fair Value, Total	832,490	875,470
Gross Unrealized (Losses), Less than 12 Months	(15,038)	(4,219)
Gross Unrealized (Losses), 12 Months or More	(14,823)	(8,589)
Gross Unrealized (Losses), Total	(29,861)	(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	968,691	755,013
Estimated Fair Value, 12 Months or More	497,066	534,955
Estimated Fair Value, Total	1,465,757	1,289,968
Gross Unrealized (Losses), Less than 12 Months	(25,537)	(7,668)
Gross Unrealized (Losses), 12 Months or More	(26,639)	(16,591)
Gross Unrealized (Losses), Total	(52,176)	(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	280,890	230,898
Estimated Fair Value, 12 Months or More	59,159	55,288
Estimated Fair Value, Total	340,049	286,186
Gross Unrealized (Losses), Less than 12 Months	(5,803)	(1,719)
Gross Unrealized (Losses), 12 Months or More	(2,062)	(1,261)
Gross Unrealized (Losses), Total	$ (7,865)	$ (2,980)