Fair Value (Fair Value By Level) (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Assets:
Available for sale debt securities	$ 3,509,896,000	$ 4,124,185,000
Equity Securities	6,013,000	27,885,000
Marketable Securities	3,518,309,000	4,154,470,000
Assets/liabilities measured at fair value
Assets:
Available for sale debt securities	3,509,896,000
Assets/liabilities measured at fair value \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,320,727,000	3,891,233,000
Assets/liabilities measured at fair value \| Obligations of states and political subdivisions
Assets:
Available for sale debt securities	189,169,000	232,951,000
Assets/liabilities measured at fair value \| Equity securities
Assets:
Equity Securities		27,886,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value
Assets:
Marketable Securities	3,515,909,000	4,152,070,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,320,727,000	3,891,233,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Obligations of states and political subdivisions
Assets:
Available for sale debt securities	189,169,000	232,951,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Equity securities
Assets:
Equity Securities	6,013,000	27,886,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable Securities	6,013,000	27,886,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Assets:
Equity Securities	6,013,000	27,886,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Significant Other Observable Inputs (Level 2)
Assets:
Marketable Securities	3,509,896,000	4,124,184,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Significant Other Observable Inputs (Level 2) \| Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,320,727,000	3,891,233,000
Measured on a recurring basis: \| Assets/liabilities measured at fair value \| Significant Other Observable Inputs (Level 2) \| Obligations of states and political subdivisions
Assets:
Available for sale debt securities	$ 189,169,000	$ 232,951,000

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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Amount of investment in marketable security.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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