Estimated Fair Values (Assets and Liabilities Carried at Fair Value on a Recurring Basis Significant Unobservable Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment securities available for sale	$ 4,783,507	$ 4,783,507	$ 7,180,180
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	59,653	0
Transfers in		59,653
Amounts included in net income	48	48
Unrealized net (losses) gains included in other comprehensive income	(97)	(97)
Purchases	51,591	51,591
Sales	(2,405)	(2,405)
Ending balance	108,790	108,790
Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		0
Transfers in		5,618
Amounts included in net income		7
Unrealized net (losses) gains included in other comprehensive income		18
Purchases		0
Sales		0
Ending balance	$ 5,643	$ 5,643

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-SubTopic 10
-Section 50
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- Definition

Amount of gain (loss) recognized in other comprehensive income, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

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