Investment Securities - Amortized Cost and Fair Values of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 422,096	$ 372,513
Gross Unrealized Gains	345	60
Gross Unrealized Losses	(4,192)	(3,955)
Estimated Fair Value	418,249	368,618
Obligations of U.S. Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	40,540	38,667
Gross Unrealized Gains	335	55
Gross Unrealized Losses	(161)	(721)
Estimated Fair Value	40,714	38,001
Foreign Government Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	200	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	200	0
Mortgage - Backed Securities - Residential [Member] \| US Government Agencies Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,925	4,297
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(14)	(4)
Estimated Fair Value	3,911	4,293
Mortgage - Backed Securities - Residential [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	305,126	280,430
Gross Unrealized Gains	10	5
Gross Unrealized Losses	(2,552)	(2,527)
Estimated Fair Value	302,584	277,908
Mortgage - Backed Securities - Commercial [Member] \| US Government Agencies Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	58,249	34,444
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,394)	(561)
Estimated Fair Value	56,855	33,883
Mortgage - Backed Securities - Commercial [Member] \| Obligations of U.S. Government-Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	14,056	14,675
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(71)	(142)
Estimated Fair Value	$ 13,985	$ 14,533

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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