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, 2017	Dec. 31, 2016
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,665,745	$ 2,033,477
Estimated Fair Value, 12 Months or More	103,547	114,975
Estimated Fair Value, Total	1,769,292	2,148,452
Gross Unrealized (Losses), Less than 12 Months	(18,203)	(26,714)
Gross Unrealized (Losses), 12 Months or More	(3,087)	(3,793)
Gross Unrealized (Losses), Total	(21,290)	(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	6,701	9,420
Estimated Fair Value, 12 Months or More	25,788	33,248
Estimated Fair Value, Total	32,489	42,668
Gross Unrealized (Losses), Less than 12 Months	(84)	(142)
Gross Unrealized (Losses), 12 Months or More	(446)	(783)
Gross Unrealized (Losses), Total	(530)	(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	7,166	20,539
Estimated Fair Value, 12 Months or More	1,315	654
Estimated Fair Value, Total	8,481	21,193
Gross Unrealized (Losses), Less than 12 Months	(26)	(135)
Gross Unrealized (Losses), 12 Months or More	(15)	(5)
Gross Unrealized (Losses), Total	(41)	(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	49,320	43,615
Estimated Fair Value, 12 Months or More	2,359	222
Estimated Fair Value, Total	51,679	43,837
Gross Unrealized (Losses), Less than 12 Months	(873)	(822)
Gross Unrealized (Losses), 12 Months or More	(98)	(6)
Gross Unrealized (Losses), Total	(971)	(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	731,409	588,352
Estimated Fair Value, 12 Months or More	41	0
Estimated Fair Value, Total	731,450	588,352
Gross Unrealized (Losses), Less than 12 Months	(7,632)	(10,582)
Gross Unrealized (Losses), 12 Months or More	(1)	0
Gross Unrealized (Losses), Total	(7,633)	(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	755,575	1,127,501
Estimated Fair Value, 12 Months or More	72,249	76,196
Estimated Fair Value, Total	827,824	1,203,697
Gross Unrealized (Losses), Less than 12 Months	(8,683)	(12,722)
Gross Unrealized (Losses), 12 Months or More	(2,520)	(2,909)
Gross Unrealized (Losses), Total	(11,203)	(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	115,574	244,050
Estimated Fair Value, 12 Months or More	1,795	4,655
Estimated Fair Value, Total	117,369	248,705
Gross Unrealized (Losses), Less than 12 Months	(905)	(2,311)
Gross Unrealized (Losses), 12 Months or More	(7)	(90)
Gross Unrealized (Losses), Total	$ (912)	$ (2,401)