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, 2017	Dec. 31, 2016
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,554,184	$ 2,033,477
Estimated Fair Value, 12 Months or More	1,022,055	114,975
Estimated Fair Value, Total	2,576,239	2,148,452
Gross Unrealized (Losses), Less than 12 Months	(14,150)	(26,714)
Gross Unrealized (Losses), 12 Months or More	(27,986)	(3,793)
Gross Unrealized (Losses), Total	(42,136)	(30,507)
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	5,214	9,420
Estimated Fair Value, 12 Months or More	29,432	33,248
Estimated Fair Value, Total	34,646	42,668
Gross Unrealized (Losses), Less than 12 Months	(113)	(142)
Gross Unrealized (Losses), 12 Months or More	(687)	(783)
Gross Unrealized (Losses), Total	(800)	(925)
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	19,345	20,539
Estimated Fair Value, 12 Months or More	3,874	654
Estimated Fair Value, Total	23,219	21,193
Gross Unrealized (Losses), Less than 12 Months	(80)	(135)
Gross Unrealized (Losses), 12 Months or More	(33)	(5)
Gross Unrealized (Losses), Total	(113)	(140)
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	37,304	43,615
Estimated Fair Value, 12 Months or More	29,446	222
Estimated Fair Value, Total	66,750	43,837
Gross Unrealized (Losses), Less than 12 Months	(351)	(822)
Gross Unrealized (Losses), 12 Months or More	(825)	(6)
Gross Unrealized (Losses), Total	(1,176)	(828)
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	506,410	588,352
Estimated Fair Value, 12 Months or More	369,060	0
Estimated Fair Value, Total	875,470	588,352
Gross Unrealized (Losses), Less than 12 Months	(4,219)	(10,582)
Gross Unrealized (Losses), 12 Months or More	(8,589)	0
Gross Unrealized (Losses), Total	(12,808)	(10,582)
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	755,013	1,127,501
Estimated Fair Value, 12 Months or More	534,955	76,196
Estimated Fair Value, Total	1,289,968	1,203,697
Gross Unrealized (Losses), Less than 12 Months	(7,668)	(12,722)
Gross Unrealized (Losses), 12 Months or More	(16,591)	(2,909)
Gross Unrealized (Losses), Total	(24,259)	(15,631)
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	230,898	244,050
Estimated Fair Value, 12 Months or More	55,288	4,655
Estimated Fair Value, Total	286,186	248,705
Gross Unrealized (Losses), Less than 12 Months	(1,719)	(2,311)
Gross Unrealized (Losses), 12 Months or More	(1,261)	(90)
Gross Unrealized (Losses), Total	$ (2,980)	$ (2,401)