Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 210,074	$ 210,074	$ 1,506,669
Estimated Fair Value, 12 Months or More	135,879	135,879	284,538
Estimated Fair Value, Total	345,953	345,953	1,791,207
Gross Unrealized (Losses), Less than 12 Months		(490)	(12,487)
Gross Unrealized (Losses), 12 Months or More		(1,533)	(7,772)
Gross Unrealized (Losses), Total		(2,023)	(20,259)
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	10,548	10,548	18,924
Estimated Fair Value, 12 Months or More	30,728	30,728	30,591
Estimated Fair Value, Total	41,276	41,276	49,515
Gross Unrealized (Losses), Less than 12 Months		(86)	(81)
Gross Unrealized (Losses), 12 Months or More		(421)	(653)
Gross Unrealized (Losses), Total		(507)	(734)
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	7,495	7,495	4,289
Estimated Fair Value, 12 Months or More	969	969	2,842
Estimated Fair Value, Total	8,464	8,464	7,131
Gross Unrealized (Losses), Less than 12 Months		(13)	(12)
Gross Unrealized (Losses), 12 Months or More		(2)	(20)
Gross Unrealized (Losses), Total		(15)	(32)
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	9,072	9,072	20,300
Estimated Fair Value, 12 Months or More	249	249	1,863
Estimated Fair Value, Total	9,321	9,321	22,163
Gross Unrealized (Losses), Less than 12 Months		(50)	(222)
Gross Unrealized (Losses), 12 Months or More		(4)	(16)
Gross Unrealized (Losses), Total		(54)	(238)
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	100,191	100,191	82,177
Estimated Fair Value, 12 Months or More	0	0	0
Estimated Fair Value, Total	100,191	100,191	82,177
Gross Unrealized (Losses), Less than 12 Months	(166)		(313)
Gross Unrealized (Losses), 12 Months or More	0		0
Gross Unrealized (Losses), Total	(166)		(313)
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	72,256	72,256	1,135,533
Estimated Fair Value, 12 Months or More	98,118	98,118	238,152
Estimated Fair Value, Total	170,374	170,374	1,373,685
Gross Unrealized (Losses), Less than 12 Months		(154)	(8,832)
Gross Unrealized (Losses), 12 Months or More		(998)	(6,929)
Gross Unrealized (Losses), Total		(1,152)	(15,761)
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	10,512	10,512	238,668
Estimated Fair Value, 12 Months or More	5,815	5,815	11,090
Estimated Fair Value, Total	$ 16,327	16,327	249,758
Gross Unrealized (Losses), Less than 12 Months		(21)	(2,902)
Gross Unrealized (Losses), 12 Months or More		(108)	(154)
Gross Unrealized (Losses), Total		$ (129)	(3,056)
Asset-Backed Securities and Structured Financial Products [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months			6,778
Estimated Fair Value, 12 Months or More			0
Estimated Fair Value, Total			6,778
Gross Unrealized (Losses), Less than 12 Months			(125)
Gross Unrealized (Losses), 12 Months or More			0
Gross Unrealized (Losses), Total			$ (125)