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	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 79,450	$ 1,506,669
Estimated Fair Value, 12 Months or More	264,317	284,538
Estimated Fair Value, Total	343,767	1,791,207
Gross Unrealized (Losses), Less than 12 Months	(301)	(12,487)
Gross Unrealized (Losses), 12 Months or More	(2,377)	(7,772)
Gross Unrealized (Losses), Total	(2,678)	(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	17,160	18,924
Estimated Fair Value, 12 Months or More	25,951	30,591
Estimated Fair Value, Total	43,111	49,515
Gross Unrealized (Losses), Less than 12 Months	(100)	(81)
Gross Unrealized (Losses), 12 Months or More	(395)	(653)
Gross Unrealized (Losses), Total	(495)	(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	2,391	4,289
Estimated Fair Value, 12 Months or More	975	2,842
Estimated Fair Value, Total	3,366	7,131
Gross Unrealized (Losses), Less than 12 Months	(3)	(12)
Gross Unrealized (Losses), 12 Months or More	(3)	(20)
Gross Unrealized (Losses), Total	(6)	(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	3,000	20,300
Estimated Fair Value, 12 Months or More	1,745	1,863
Estimated Fair Value, Total	4,745	22,163
Gross Unrealized (Losses), Less than 12 Months	(9)	(222)
Gross Unrealized (Losses), 12 Months or More	(13)	(16)
Gross Unrealized (Losses), Total	(22)	(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	0	82,177
Estimated Fair Value, 12 Months or More	1,054	0
Estimated Fair Value, Total	1,054	82,177
Gross Unrealized (Losses), Less than 12 Months	0	(313)
Gross Unrealized (Losses), 12 Months or More	(2)	0
Gross Unrealized (Losses), Total	(2)	(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	47,252	1,135,533
Estimated Fair Value, 12 Months or More	213,510	238,152
Estimated Fair Value, Total	260,762	1,373,685
Gross Unrealized (Losses), Less than 12 Months	(133)	(8,832)
Gross Unrealized (Losses), 12 Months or More	(1,805)	(6,929)
Gross Unrealized (Losses), Total	(1,938)	(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	9,647	238,668
Estimated Fair Value, 12 Months or More	21,082	11,090
Estimated Fair Value, Total	30,729	249,758
Gross Unrealized (Losses), Less than 12 Months	(56)	(2,902)
Gross Unrealized (Losses), 12 Months or More	(159)	(154)
Gross Unrealized (Losses), Total	$ (215)	(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)